RUBBER-CONTENT  OF  NORTH  AMERICAN  PLANTS 


UC-NRLF 


B  M   S73 


BY 


HARVEY  M.  HALL  AND  FRANCES  L.  LONG 


PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
WASHINGTON,  1921 


UNIVERSITY  FARM 


SB289 


RUBBER-CONTENT  OF  NORTH  AMERICAN  PLANTS 


BY 


HARVEY  M.  HALL  AND  FRANCES  L.  LONG 


PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
WASHINGTON,  1921 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
PUBLICATION  No.  313 


PRESS  OP  GIBSON  BROTHERS,  INC. 
WASHINGTON,  D.  C. 


CONTENTS. 


PAGE. 

I.  Purpose  of  the  investigation 5 

II.  Earlier  investigations 6 

III.  Acknowledgments 12 

IV.  Methods  employed 12 

Collection  and  preparation  of  samples 12 

Chemical  analysis 13 

V.  The  most  important  species 15 

The  genus  Asclepias,  or  true  milkweeds 15 

Asclepias  subulata  (desert  milkweed) 17 

Asclepias  sullivanti 22 

Asclepias  syriaca  (common  milkweed) 24 

Asclepias  californica 26 

Asclepias  latifolia  (broad-leaf  milkweed) 29 

Asclepias  mexicana 31 

Asclepias  galioides  (whorled  milkweed) 33 

Asclepias  brachystephana 34 

Asclepias  speciosa  (showy  milkweed) 35 

Apocynum  cannabinum  (Indian  hemp)  and  A.  androssemifolium  (spread- 
ing dogbane) 37 

VI.  Miscellaneous  species 39 

Acerates  auriculata 39 

Asclepias  eriocarpa 39 

Asclepiodora  decumbens 40 

Cryptostegia  grandiflora 41 

Jatropha  cardiophylla 41 

Hymenoxys  floribunda  utilis 41 

VII.  Species  with  very  low  rubber-content 42 

VIII.  Species  examined  with  negative  results 46 

IX.  Variation  in  the  rubber-content  of  Asclepias  and  Apocynum 47 

Distribution  of  rubber  in  the  plant 47 

Variation  due  to  heredity  and  to  environment 48 

Seasonal  variation 50 

Variation  in  successive  crops 52 

X.  Properties  of  the  rubbers 54 

XI.  By-products  from  Asclepias  and  Apocynum 56 

XII.  Agricultural  possibilities  of  Asclepias  and  Apocynum 60 

XIII.  Summary  and  conclusions 63 

XIV.  Literature  cited 65 

3 


RUBBER-CONTENT  OF  NORTH  AMERICAN  PLANTS, 


I.  PURPOSE  OF  THE  INVESTIGATION. 

Rubber  is  now  considered  by  every  civilized  community  as  one  of 
its  prime  necessities.  Since  this  important  commodity  is  almost 
wholly  a  product  of  the  tropics,  those  countries  which  lie  entirely  or 
mostly  in  the  temperate  zones  are  dependent  upon  importations,  mostly 
from  overseas,  for  their  supply.  This  places  the  importing  nation 
at  a  disadvantage,  since  it  is  subject  at  all  times  to  labor  and  other 
economic  disturbances  in  the  rubber-producing  countries,  while  in 
time  of  war  it  is  dependent  upon  the  good-will  both  of  the  producers 
and  of  whatever  power  happens  at  the  moment  to  be  in  control  of  those 
seas  over  which  the  importations  must  travel.  The  United  States  is 
in  a  particularly  hazardous  position  in  this  respect.  The  normal 
annual  importation  into  this  country  is  approximately  300,000,000 
pounds.  War-time  conditions  would  make  such  increased  and  impera- 
tive demands  that,  unless  this  amount  could  be  very  materially 
augmented,  it  would  be  necessary  greatly  to  restrict  the  use  of  rubber  for 
ordinary  commercial  purposes.  This  inconvenience,  however,  would 
be  trivial  as  compared  with  the  perilous  condition  of  the  country  in 
case  importations  from  overseas  should  at  the  same  time  be  seriously 
curtailed  or  stopped  entirely  by  the  enemy.  It  therefore  behooves 
the  United  States  and  other  countries  so  situated  to  discover  some  other 
method  of  supplying  their  needs,  and  it  is  possible  that  one  such  method 
lies  in  the  improvement  and  cultivation  of  rubber  plants  suited  to 
extra-tropical  conditions.  The  responsibility  for  the  initiation  of  such 
studies  lies  primarily  with  the  botanist,  since  he  is  the  one  who  should 
be  the  best  qualified  to  recognize  the  kinds  of  plants  most  likely  to 
yield  results.  It  was  with  these  considerations  in  mind  that  the 
present  investigation  was  undertaken,  although  the  general  scientific 
interest  which  attaches  to  the  formation  and  presence  of  rubber  hi 
plants  was  also  an  impelling  motive. 

The  work  described  in  this  paper  is  to  some  extent  a  continuation 
of  a  survey  of  western  North  America  for  rubber-producing  plants 
begun  in  1917  as  a  war-emergency  project  under  the  State  Council  of 
Defense  of  California  and  of  which  there  is  more  specific  reference  on 
p.  8.  That  project,  however,  centered  around  a  study  of  the  genus 
Chrysothamnus  and  related  shrubs,  plants  in  which  the  rubber  occurs 
in  the  form  of  solid  particles  in  the  plant  cell,  whereas  the  present 
studies  are  concerned  chiefly  with  the  genus  Asckpias  (milkweeds) 
and  similar  herbaceous  plants  in  which  the  rubber  occurs  somewhat 
as  an  emulsion  in  the  latex,  or  milky  sap,  which  in  turn  occupies  the 
vessels  of  the  highly  specialized  laticiferous  tissues.  Furthermore, 


6  KUBBER-CONTENT  OF   NORTH   AMERICAN   PLANTS. 

while  the  prime  object  of  that  earlier  search  was  to  locate  an  emergency 
supply  of  rubber  in  native  shrubs,  the  object  of  the  present  one  is  to 
discover,  if  possible,  some  latex  plant  sufficiently  promising  to  justify 
experiments  looking  towards  its  cultivation  as  a  rubber-producing 
crop.  A  second  object  is  to  extend  the  knowledge  of  the  occurrence 
of  rubber  in  plants,  quite  irrespective  of  any  immediate  practical 
results.  During  both  of  these  investigations  attention  has  been  also 
given  to  many  other  plants  than  those  of  the  two  genera  just  men- 
tioned, so  that  definite  information  is  now  available  as  to  the  presence 
or  absence  of  rubber  in  about  225  species  and  varieties  of  west  American 
plants. 

II.  EARLIER  INVESTIGATIONS. 

The  most  important  studies  thus  far  made  on  North  American 
rubber  plants  have  been  those  on  Castillo,  and  guayule,  the  former  a 
tree  of  tropical  America,  the  other  a  low  shrub  of  northern  Mexico 
and  southwestern  Texas.  Investigations  of  latex  plants  native  to  the 
United  States  have  been  few  and,  at  least  so  far  as  indicated  by 
published  reports,  they  resulted  in  no  promising  discoveries.  This 
was  due  in  some  cases  to  chemical  and  other  difficulties  encountered 
in  the  analyses  of  the  samples;  sometimes  to  a  lack  of  persistence  in 
assembling  samples  from  a  large  number  of  species  and  from  a  con- 
siderable variety  of  habitats;  sometimes,  and  perhaps  always  in  part, 
to  the  chance  which  resulted  in  the  wrong  botanical  species  as  the 
object  of  study  or  to  its  examination  at  an  unfavorable  season. 
One  of  the  outstanding  results  of  the  present  inquiry  is  the  discovery 
that  not  only  do  closely  related  species  vary  among  themselves  in 
their  rubber-content  to  a  surprising  degree,  but  also  that  a  remarkable 
range  of  variation  may  be  expected  within  a  single  species.  Even  the 
different  parts  of  the  plant  are  usually  found  to  carry  very  unequal 
percentages,  and  these  proportions  vary  somewhat  with  seasonal  and 
other  ecologic  conditions.  It  is  therefore  evident  that,  if  one  sets 
out  to  discover  the  best  native  rubber-producing  species,  he  should  not 
rest  content  until  a  very  large  series  of  examinations  has  been  macle. 
It  will  be  seen  from  the  following  summaries  of  the  results  obtained  by 
earlier  workers  that  with  the  exception  of  Castillo,  and  guayule  the 
investigations  were  not  carried  far  enough  to  give  an  adequate  basis 
for  judgment,  and  that  in  a  number  of  cases  the  plants  deemed  as 
unworthy  of  further  study  have  since  been  shown  to  possess  consider- 
able promise  as  rubber  plants. 

Central  American  Rubber  Tree  (Castilla  elastica). — Since  this  is  a  trop- 
ical plant  not  suitable  for  cultivation  in  the  United  States,  it  will  here 
receive  but  the  briefest  mention,  and  that  only  for  the  sake  of  making 
the  list  as  nearly  complete  as  possible.  The  most  authoritative 
account  of  the  plant  is  one  by  Cook  (1903).  The  conclusions  of  this 


EARLIER   INVESTIGATIONS.  7 

author  are  that  the  culture  of  Castillo,  has  passed  the  experimental 
stage,  but  he  emphasizes  the  fact  that  the  regions  in  Mexico  and  Central 
America  well  adapted  to  its  culture  are  much  more  limited  than 
generally  supposed.  With  no  attempt  to  discourage  rubber  cultiva- 
tion in  those  districts,  he  nevertheless  urges  caution  in  making  invest- 
ments. The  rubber,  which  is  carried  by  a  latex  obtained  by  the  tap- 
ping process,  is  scarcely  inferior  to  Para  rubber  and  is  obtained 
in  some  quantity  from  native  trees  in  Central  America. 

Other  Mexican  latex  plants. — Mexico  is  a  very  promising  field  for 
exploration  with  the  object  of  discovering  plants  capable  of  yielding 
rubber  in  commercial  quantities,  both  from  the  native  growth  and 
from  plantations  established  for  this  purpose.  This  was  the  conviction 
of  the  late  Dr.  Pehr  Olsson-Seffer,  who,  more  than  anyone  else,  had 
looked  into  the  matter  as  an  experienced  student  of  rubber  plants. 
In  fact,  Olsson-Seffer  was  preparing  to  take  advantage  of  this  knowl- 
edge when  political  disturbances  in  Mexico,  and  finally 'his  own  death 
in  connection  with  one  of  these,  put  a  stop  to  his  plans.  The  plants 
which  he  considered  as  of  greatest  value  are  species  of  Jatropha, 
Plumeria,  and  Euphorbia  which  have,  in  addition  to  their  qualities 
as  rubber  producers,  the  very  desirable  ability  to  reproduce  from  the 
base  when  the  tops  are  removed,  thus  affording  a  perennial  supply 
and  avoiding  the  great  expense  of  tapping  by  hand.  The  results  of 
Olsson-Seffer's  studies  have  not  been  published  in  detail,  although 
some  reference  has  been  made  to  them  by  Dunstan  (1910)  in  an  article 
in  which  it  is  said  that  some  species  of  Plumeria  yield  a  latex  averaging 
14  to  16  per  cent  and  sometimes  with  as  much  as  24  per  cent  of  rubber, 
while  the  latex  of  Euphorbia  calyculata  yields  21  per  cent  of  rubber  of 
good  quality. 

A  Mexican  plant  which  has  received  some  attention  as  a  source  of 
rubber  is  the  palo  amarillo  (Euphorbia  fulva  Stapf ,  E.  elastica  Alta- 
mirano  and  Rose,  not  Jumelle).  The  best  account  of  this  species  has 
been  given  by  Rusby  (1909),  who  states  that  it  is  a  tree  of  the  sub- 
tropical region,  and  especially  of  the  hilly  country,  where  the  western 
edge  of  the  Mexican  table-land  breaks  down  into  the  coastal  slope, 
at  an  altitude  mostly  of  5,000  to  7,000  feet.  It  belongs  principally 
to  the  States  of  Michoacan,  Guanajuato,  and  Jalisco.  The  latex  is 
obtained  by  tapping  and  that  from  the  stems  carries  7.3  to  15.7  per  cent 
of  caoutchouc.  The  rubber  is  about  equal  in  value  to  that  of  guayule, 
but  the  amount  collected  has  been  so  small  that  it  has  not  entered  into 
commerce  to  any  appreciable  extent.  The  chief  interest  in  this  plant 
lies  in  its  possibilities  if  properly  developed.  It  is  very  easily  propa- 
gated by  cuttings  and  full-sized  trees  are  thus  obtained  in  5  to  7  years. 

Other  Mexican  species  mentioned  by  Dunstan  (1910)  as  containing 
small  amounts  of  rubber  are  Jatropha  wrens,  Pedilanthus  tomentellus, 
and  P.  pringki. 


8  RUBBER-CONTENT   OF  NORTH   AMERICAN   PLANTS. 

Guayule  (Parthenium  argentatum). — The  most  extensive  use  thus  far 
made  of  North  American  rubber  plants  has  been  that  of  guayule,  a 
Mexican  and  Texan  shrub  that  came  into  prominence  about  1902. 
This  plant  was  formerly  very  abundant  in  some  parts  of  Mexico  and 
the  value  of  the  exported  product  exceeded  $10,000,000  in  1911.  In 
recent  years,  however,  the  production  has  fallen  off  very  markedly. 
As  the  native  supply  began  to  wane,  attempts  were  made  to  bring  the 
species  under  cultivation,  at  first  with  but  little  success  because  of 
difficulties  encountered  in  the  germination  of  the  seed.  It  was  also 
found  that  whereas  the  native  shrub  yielded  an  average  of  10  per  cent 
of  guayule  gum,  or  "rubber,"  the  content  of  the  cultivated  plants  fell 
in  some  cases  to  less  than  2  per  cent.  After  considerable  experimenta- 
tion a  high  rate  of  germination  was  finally  obtained,  and  through  the 
segregation  and  selection  of  superior  strains  the  content  of  field-grown 
plants  is  said  to  have  been  increased  to  18  per  cent.  In  fact,  a  few 
strains  carrying  a  considerably  higher  percentage  have  been  developed, 
but  it  is  understood  that  these  have  certain  characteristics  which 
render  them  unsuitable  for  commercial  planting.  Whether  the  18 
per  cent  just  mentioned  is  for  pure  rubber  or  whether  it  includes  a 
considerable  amount  of  resin  and  other  impurities,  as  in  commercial 
guayule,  is  not  known  to  the  writers.  It  is  now  well  established  that 
the  wild  plants  belong  to  a  series  of  innumerable  races,  that  these 
vary  to  a  considerable  extent,  and  that  reproduction  is  partheno- 
genetic,  at  least  in  part.  The  method  of  improvement,  therefore, 
has  been  to  select  the  best  strains  as  determined  by  analysis  and 
cultural  characters  and  to  propagate  from  these  plants  without  the 
introduction  of  cross-breeding.  The  rubber  in  guayule  occurs  as 
minute  bodies  in  the  cells,  there  being  no  latex  as  in  Hevea,  Ficus,  and 
most  other  rubber  plants.  The  planting  of  guayule  on  a  large  scale 
has  been  started  in  southern  Arizona  under  the  scientific  direction  of 
Maccallum,  but  the  success  of  the  industry  is  still  a  matter  for  the 
future  to  decide.  The  only  extensive  account  of  rubber  as  it  occurs 
in  guayule  has  been  presented  by  Lloyd  (1911).  This  author  describes 
field  conditions  and  factory  methods,  in  addition  to  the  results  of  his 
histological  investigations  on  the  formation  and  distribution  of  rubber 
in  the  plant. 

Chrysil  and  related  rubbers. — Immediately  upon  the  entrance  of  the 
United  States  into  the  great  war  in  1917,  the  State  of  Calif ornia  under- 
took a  survey  of  the  Great  Basin  and  Pacific  Coast  areas  for  rubber- 
producing  plants,  the  work  being  carried  out  under  the  direction  of  the 
State  Council  of  Defense  and  the  University  of  California.  The 
results  have  been  published  by  Hall  and  Goodspeed  (1919).  Accord- 
ing to  these  authors,  rubber  of  good  quality  has  been  found  in  12 
varieties  of  Chrysothamnus  nauseosus  or  rabbit-brush,  the  best  shrubs 


EARLIER   INVESTIGATIONS.  9 

carrying  as  high  as  6.5  per  cent,  while  the  average  is  about  2.5  per  cent. 
These  figures  are  for  rubber  obtained  by  the  acetone-benzene  method. 
In  commercial  extraction,  such  as  is  practiced  for  guayule,  the  amount 
would  be  increased  because  of  the  inclusion  of  anywhere  from  10  to 
25  per  cent  of  resins  and  other  impurities.  The  pure  product  was 
given  the  name  of  chrysil.  A  total  of  300,000,000  pounds  was  esti- 
mated to  be  present  in  the  native  plants.  Rubber  was  also  found  in 
various  amounts  in  4  other  species  of  Chrysothamnus  and  in  10  species 
of  the  closely  related  genus,  Haplopappus.  Since  the  prime  object 
was  to  locate  an  emergency  supply  of  native  rubber,  the  investigations 
were  not  extended  to  include  cultural  experiments,  although  some 
information  is  presented  that  could  be  used  as  a  basis  for  such  work 
and  as  a  beginning  in  breeding  and  selection.  Chrysil  is  not  a  latex 
rubber,  but  occurs  as  solid  particles  in  the  plant  cells,  much  after  the 
manner  of  guayule,  as  is  indicated  by  detailed  histological  studies,  the 
results  of  which  are  presented  by  these  authors. 

Colorado  rubber  plant. — In  1902  and  1903  considerable  interest 
was  manifested  in  a  new  rubber  plant  from  Colorado  and  New  Mexico, 
namely,  the  pinguay  (Hymenoxys  floribunda  utilis)  or  Colorado  rubber 
plant,  as  it  soon  came  to  be  called.  This  was  before  the  time  of  plan- 
tation rubber  and  when  the  crude  product  from  the  upper  Amazon 
districts  commanded  high  prices  because  of  the  rapidly  increasing 
demand.  Small  mills  were  erected  in  southern  Colorado  and  experi- 
ments were  undertaken  in  the  cultivation  of  the  plant.  It  was  soon 
learned,  however,  that  profits  could  not  be  made  by  utilizing  the  wild 
plants,  owing  to  their  small  size  and  scattered  growth,  and  that  the 
plants  did  not  yield  enough  rubber  to  make  then-  cultivation  profitable. 
Very  little  has  been  published  concerning  this  plant,  but  there  is  a 
note  by  T.  D.  A.  Cockerell  (1903)  and  another  by  W.  P.  Cockerell 
(1904).  The  latter  states  that  the  roots  are  said  to  contain  5  to  12 
per  cent  of  crude  rubber,  but  apparently  this  includes  a  considerable 
proportion  of  impurities.  Recent  analyses  have  yielded  only  3.6  per 
cent  of  pure  rubber  in  the  root  and  basal  portions  of  the  plant  (p.  41). 
Hillier  (1906)  expresses  the  opinion  that  the  product  does  not  compare 
favorably  with  many  of  the  lower  grades  of  rubber  already  on  the 
market. 

Milkweeds  (Asclepias  spp.)  and  Indian  hemp  (Apocynum  cannabinum). — 
The  first  recorded  attempt  to  prepare  rubber  from  any  of  the  plants 
which  grow  north  of  the  Mexican  boundary  was  that  of  William 
Saunders  (1875),  who  experimented  with  the  common  milkweed 
(Asclepias  syriaca  =  A.  cornuti).  The  investigations  were  carried  on 
about  1871  at  London,  Ontario.  Saunders  reported  the  preparation  of 
an  elastic,  vulcanizable  gum  which  he  thought  could  be  manufactured 
at  a  profit.  By  allowing  the  coarse-ground  material  to  ferment  for 


10  RUBBER-CONTENT   OF   NORTH   AMERICAN   PLANTS. 

several  days,  drying,  then  adding  carbon  bisulphide,  he  was  able  to 
obtain  nearly  5  per  cent  of  this  substance,  which  evidently  was  a 
mixture  of  rubber  and  various  other  ingredients.  No  serious  attempt 
was  made  to  commercialize  this  discovery.  A.  T.  Saunders  (1910) 
reports  the  results  of  examinations  made  in  1900  of  a  milkweed  the 
species  of  which  is  not  given.  Only  a  very  small  amount  of  rubber  was 
prepared,  but  the  physical  properties  are  described  as  indicated 
later  (p.  54). 

Apparently  the  most  extensive  examination  of  the  milkweeds 
hitherto  made  was  that  of  Fox  (1911),  who  carried  on  his  experiments 
at  Akron,  Ohio.  This  worker  obtained  2  to  3  per  cent  of  rubber  from 
Asclepias  syriaca  "on  the  basis  of  the  latex."  His  conclusions  were 
that  "while  rubber  is  a  product  of  the  plant,  the  amount  is  so  small, 
its  quality  is  so  inferior,  and  its  cost  of  production  is  so  high,  that  a 
profitable  industry  is  out  of  the  question."  A  year  later  Fox  (1912) 
reported  more  favorably  concerning  his  work  on  Apocynum  cannabinum 
or  Indian  hemp.  The  latex  of  this  species  was  found  to  contain  only 
1.12  to  2.36  per  cent  of  rubber,  but  the  quality  of  the  product  was  much 
superior  to  that  obtained  from  Asclepias.  Other  characters  of  the 
rubber  are  given  on  p.  55)  It  is  to  the  credit  of  this  investigator  that 
he  carried  his  examination  so  far,  although  working  with  plants 
which  gave  such  discouragingly  low  yields. 

Widtsoe  and  Hirst,  working  at  the  Utah  Agricultural  College,  made 
percentage  determinations  for  several  species  and  prepared  a  small 
amount  of  rubber  for  examination.  The  method  consisted  in  extrac- 
tion with  benzene  in  a  Soxhlet  apparatus  and  precipitation  with 
alcohol.  The  precipitate  was  in  some  cases  treated  with  acetone  to 
remove  fats,  etc.  The  best  result  was  obtained  with  Asclepias  speciosa, 
the  leaves  of  which  yielded  about  2.25  per  cent  by  this  process.  A. 
incarnata  gave  3.15  per  cent,  but  this  includes  at  least  a  portion  of  the 
fats  and  resins.  Asclepiodora  decumbens  yielded  about  1  per  cent, 
while  no  rubber  was  obtained  from  Apocynum  cannabinum  and  Lactuca 
scariola.  These  results  are  here  given  from  a  manuscript  report  with 
the  consent  of  the  investigators. 

Miscellaneous  reports. — It  is  reported  by  Pearson  (1916)  that  during 
the  recent  war  the  Germans,  being  unable  to  import  freely,  resorted 
to  the  manufacture  of  rubber  from  certain  weeds,  one  of  which, 
Sonchus  oleraceus,  or  common  sow-thistle,  grows  wild  as  an  introduced 
species  in  the  United  States.  Apparently  the  most  important  weed 
used  was  a  sort  of  wild  lettuce,  or  Lactuca. 

No  other  accounts  of  studies  on  latex  rubber  in  North  American 
plants  have  been  found  in  the  literature,  although  it  is  possible  that 
such  exist.  At  any  rate,  the  results  did  not  indicate  a  commercial  value 
nor  did  they  lead  to  extensive  experiments  in  the  improvement  of 
the  plants.  Not  infrequently,  however,  erroneous  reports  of  the  dis- 


EARLIER   INVESTIGATIONS.  11 

covery  of  new  rubber  plants  find  their  way  into  the  literature.  These 
reports  sometimes  cause  considerable  trouble  through  the  time  con- 
sumed in  establishing  their  falsity,  and  in  some  instances  they  have 
been  used  to  lure  investors  into  fraudulent  commercial  enterprises. 
The  more  persistent  of  these  reports  refer  to  the  following  plants : 

Ocotillo  (Fouquieria  spkndens'),  a  tall  shrub  of  the  southwestern 
deserts,  has  been  reported  a  number  of  times  as  a  rubber  plant  and 
money  invested  in  its  utilization  for  this  purpose.  However,  very 
careful  chemical  examination  shows  beyond  doubt  that  no  rubber  is 
present.  Analysis  by  the  usual  method  as  described  later  gives  no 
return  in  the  benzene  extract.  Furthermore,  when  the  pulverized 
material  is  extracted  with  benzene,  the  residue  after  evaporation  is 
entirely  soluble  in  acetone,  thus  indicating  again  that  no  rubber  is 
present.  Ocotillo  contains  considerable  amounts  of  waxes,  resins,  and 
similar  substances,  some  of  which  are  now  being  manufactured  on  a 
commercial  scale,  according  to  Pearson  (1920). 

Various  species  of  cactus  are  stated  to  contain  rubber,  but  no 
evidence  of  its  presence  has  been  found  and  the  chemical  nature  of  the 
plants  does  not  encourage  its  expectation.  Opuntia  vulgaris  was  one 
of  the  species  mentioned,  with  the  statement  that  rubber  was  obtained 
from  it  in  Arizona.  This  plant  grows  nowhere  hi  the  Southwest. 

Claims  also  have  been  made  for  certain  ocean  kelps,  especially 
Macrocystis  pyrifera,  as  rubber  plants,  and  companies  formed  in 
southern  California  to  exploit  them  on  this  basis.  A  detailed  chemical 
study  of  the  organic  constituents  of  kelps  has  been  made  by  Hoagland 
(1915),  who  reports  that  carbohydrates  or  analogous  bodies  make  up 
the  principal  portion  of  the  organic  matter  and  that  these  carbo- 
hydrates are  complex  colloidal  substances  which  would  ordinarily  be 
classed  among  the  vegetable  gums  or  pectins.  Thus  the  rubber  present, 
even  if  it  occurs  at  all,  which  seems  extremely  unlikely,  would  be  so 
small  in  amount  as  to  be  negligible. 

During  the  period  of  high  rubber  prices  much  effort  was  made  to 
find  plants  similar  to  guayule,  and  many  species  of  the  Southwest  and 
of  Mexico  were  investigated  in  this  connection.  The  one  reported 
most  frequently,  presumably  because  of  its  superficial  resemblance  to 
guayule,  together  with  its  gummy  and  resinous  nature,  was  the  brittle- 
brush  orincienso  (Encelia  farinosa) .  Repeated  analyses  of  this  plant 
prove  conclusively  that  rubber  is  not  present.  A  closer  relative  of 
guayule,  namely,  themariola  (Partheniumincanwri),  of  Mexico,  carries 
but  a  very  meager  amount  of  rubber,  according  to  Lloyd  (1911). 


12  RUBBER-CONTENT   OF   NORTH   AMERICAN   PLANTS. 

III.  ACKNOWLEDGMENTS. 

It  is  a  pleasure  to  make  grateful  acknowledgment  for  assistance 
received  from  a  number  of  people  who  have  been  sufficiently  interested 
to  gather  and  forward  samples  for  analysis,  and  in  some  cases  to  supply 
information  of  considerable  value.  The  list  of  those  who  have  con- 
tributed in  this  manner  includes  the  following:  Professor  E.  B.  Bab- 
cock,  University  of  California;  Mr.  W.  W.  Eggleston,  United  States 
Department  of  Agriculture;  Mr.  C.  L.  Forsling,  Jornada  Reserve,  Las 
Cruces,  New  Mexico;  Mr.  M.  French  Oilman,  Banning,  California; 
Mr.  C.  Hildreth,  Lincoln,  Nebraska;  Mr.  Ivan  M.  Johnston,  University 
of  California;  Professor  W.  L.  Jepson,  University  of  California;  Mr. 
C.  F.  Korstian,  United  States  Forest  Service;  Mr.  J.  V.  G.  Loftfield, 
Tucson,  Arizona;  Dr.  E.  B.  Pay  son,  Missouri  Botanical  Garden;  Mr. 
Albert  J.  Perkins,  Santa  Ana,  California;  Mrs.  C.  P.  Powell,  Berkeley, 
California;  Mr.  Carl  Purdy,  Ukiah,  California;  Miss  Inez  Sandusky, 
Wichita  Falls,  Kansas;  Mr.  E.  E.  Schellenger,  Riverside,  California; 
Dr.  Forrest  Shreve,  Carnegie  Institution  of  Washington;  Miss  Laurene 
Stevens,  Lawrence,  Kansas;  Professor  J.  J.  Thornber,  University  of 
Arizona;  Miss  H.  A.  Walker,  University  of  California;  Professor  J.  E. 
Weaver,  University  of  Nebraska.  Professor  E.  C.  McCarty,  of  the 
Colorado  Agricultural  College,  has  generously  made  a  number  of 
chemical  examinations  and  assisted  in  other  ways,  and  Mr.  H.  R.  Reed, 
of  the  United  States  Department  of  Agriculture,  not  only  collected 
numerous  samples  of  Asclepias  subulata,  but  also  carried  on  experi- 
ments with  this  plant  while  he  was  stationed  at  the  experiment  farm  at 
Bard,  California.  The  studies  have  been  carried  out  under  the  author- 
ity of  Dr.  F.  E.  Clements,  who  has  not  only  given  his  wholehearted 
support,  but  has  also  collected  many  of  the  plants  here  considered. 

IV.  METHODS  EMPLOYED. 
COLLECTION  AND  PREPARATION  OF  SAMPLES. 

Most  of  the  plants  examined  are  perennial  herbs.  Samples  of  these 
were  gathered  by  severing  the  stems  near  the  base,  or  at  about  the 
level  where  a  low-set  mowing  machine  would  cut  them.  The  cut 
ends  were  immediately  plunged  into  fine  dust  to  aid  in  the  coagula- 
tion of  the  latex  and  thus  check  the  loss  of  rubber  through  bleeding  of 
the  stems.  Pulling  was  commonly  resorted  to  in  the  case  of  annuals, 
but  the  roots  were  discarded  except  in  the  few  cases  where  the 
analysis  of  the  root  is  specifically  indicated.  This  rejection  of  the  roots 
was  practiced  because  of  the  almost  total  absence  of  rubber  below  the 
soil-line  in  latex  plants  and  also  because  the  roots  would  not  be  utilized 
in  the  preparation  of  rubber  for  commercial  purposes. 

A  wooden  drying-oven  provided  with  screen-bottom  trays  was  found 
convenient  for  purposes  of  desiccation,  the  heat  being  supplied  from 


METHODS   EMPLOYED.  13 

below.  From  12  to  24  hours  of  such  drying  at  40°  to  50°  C.  was  found 
to  be  sufficient  for  most  samples.  High  temperatures  were  avoided, 
because  of  their  tendency  to  convert  the  caoutchouc  into  resins  or 
other  products  of  oxidation.  The  moisture  still  remaining,  although 
slight,  was  determined  in  most  cases  by  the  usual  methods,  and  the 
final  analyses  were  computed  upon  the  basis  of  absolutely  dry  material. 
The  few  exceptions  are  indicated  in  the  tables. 

The  leaves  and  stems  of  most  samples  were  separated  after  drying, 
since  it  was  early  found  that  they  were  not  equally  rich  in  rubber,  and 
that  separate  analyses  were  therefore  desirable.  The  removal  of  the 
leaves  before  drying  would  have  resulted  in  the  loss  of  latex  through 
bleeding.  Leaves  and  stems  were  then  ground  separately  in  coffee 
mills,  until  nearly  all  of  the  material  would  pass  through  a  30-mesh 
brass  sieve,  i.  e.,  one  with  apertures  0.0195  inch  in  diameter.  It  was 
found  desirable  to  include  with  the  finely  ground  sample  a  very  small 
amount  ground  to  pass  only  a  20-mesh  or  10-mesh  sieve  (i.  e.,  with  aper- 
tures of  0.0335  or  of  0.0799  inch),  since  otherwise  the  sample  would 
pack  too  tightly  in  the  extractors  and  clog  the  entrance  to  the  siphons. 

CHEMICAL  ANALYSIS. 

The  acetone-benzene  method  of  extraction  was  used,  the  practice 
being  essentially  that  described  by  Hall  and  Goodspeed  (1919,  p. 
216)  in  their  work  on  chrysil.  The  Bailey-Walker  extraction  ap- 
paratus was  used,  because  of  its  simple  construction,  efficiency  as 
an  extractor,  and  the  ease  with  which  the  parts  could  be  cleaned. 
The  usual  method  was  modified  through  the  introduction  into  the 
bottom  of  the  extraction  thimble  of  a  thin  layer  of  cotton,  with  two 
disks  of  filter  paper  above  it.  About  5  grams  of  the  thoroughly  mixed 
and  ground  plant  material,  taken  from  a  weighing  bottle  with  aid  of  a 
spatula,  were  then  added.  This  filled  the  thimble  for  about  two-thirds 
of  the  way  to  the  top.  A  second  thin  layer  of  cotton  was  placed  on  top 
of  the  sample  to  prevent  danger  of  an  overflow  of  the  material  into  the 
extraction  flask.  The  extraction  itself  consists  of  two  parts,  as  follows: 

(a)  Extraction  with  10  c.  c.  of  boiling  acetone  for  3  hours  on  a 
water-bath.  This  period  was  determined  upon  after  it  had  been  found 
by  experiment  that  all  of  the  acetone-extractable  substances  came 
over  within  the  three-hour  limit.  The  first  portion  of  the  extract 
siphoned  over  is  usually  bright  green  in  color.  This  is  due  to  the 
chlorophyll,  which  is  very  readily  soluble  in  acetone.  If  the  siphon 
is  working  properly  the  chlorophyll  will  soon  be  completely  dissolved 
from  the  ground  material  and  the  acetone  extract,  as  it  passes  through 
the  siphon,  will  be  perfectly  clear.  At  the  close  of  the  three-hour 
period  the  thimble  was  removed  from  the  flask  with  blunt  forceps 
and  the  acetone  evaporated  in  a  water-jacketed  drying-oven.  The 
excess  acetone  in  the  flask  was  distilled  and  the  residue  dried  for 
4  hours  in  the  oven,  after  which  it  was  cooled  in  a  desiccator  and 
weighed.  This  residue  is  assumed  to  contain  all  of  the  chlorophyll, 


14  RUBBER-CONTENT   OF  NORTH  AMERICAN  PLANTS. 

fats,  resins,  and  similar  substances.  It  is  reported  in  the  following 
tables,  under  the  heading  of  "acetone  extract."  Since  rubber  itself  is 
partially  soluble  hi  boiling  acetone  when  fats  are  present,  it  is  possible 
that  small  amounts  are  included  in  this  extract,  but  since  the  parts 
of  the  plants  examined  presumably  contain  but  small  amounts  of  fats, 
it  is  believed  that  the  error  due  to  this  is  negligible. 

(6)  Extraction  with  10  c.  c.  of  boiling  benzene  for  3  hours,  prefer- 
ably on  an  electric  plate.  The  material  remaining  in  the  thimble 
after  the  extraction  with  acetone  as  just  described  was  thoroughly 
dried,  the  thimble  with  its  contents  was  placed  in  a  clean  flask,  and 
subjected  to  boiling  benzene.  At  the  close  of  the  extraction  the  ben- 
zene was  distilled  off  and  the  residue  dried  in  a  water-jacketed  oven, 
cooled  in  a  desiccator,  and  weighed.  This  residue  was  taken  to  be 
rubber  and  is  so  reported  in  the  following  tables,  since  all  other  com- 
pounds soluble  in  benzene  had  been  previously  removed  by  the  acetone. 
In  some  cases,  especially  those  in  which  the  percentage  of  the  benzene 
extract  ran  very  low,  the  residue  would  have  the  consistency  of  thick 
sirup  and  lacked  elasticity.  It  seems  not  unlikely  that  in  some  species 
which  carry  less  than  1  per  cent,  as  given  in  the  tables,  the  material  is 
not  really  rubber,  but  other  substances  that  escaped  extraction  by 
acetone,  perhaps  mixed  with  more  or  less  fat.  On  the  other  hand, 
when  larger  amounts  of  benzene  extract  were  obtained,  the  material 
would  possess  elasticity,  resilience,  and  other  properties  of  crude  com- 
mercial rubber.  Whe.ther  or  not  the  substance  is  in  reality  pure  rubber 
depends  somewhat  upon  the  definition  of  that  word,  but  since  the 
methods  of  analysis  used  are  those  generally  employed  by  rubber 
chemists,  and  in  consideration  of  the  physical  properties  of  the  product, 
there  is  every  reason  to  believe  that  the  percentage  obtained  in  each 
case  represents  approximately  the  amount  of  crude  rubber  present, 
and  it  is  so  reported  in  the  tables  of  analyses. 

In  regard  to  the  accuracy  of  the  results,  it  may  be  stated  that,  in 
view  of  the  partial  solubility  of  rubber  in  acetone  when  fats  are  present, 
and  because  of  the  difficulty  in  attaining  perfection  in  the  extraction 
from  plant  tissues,  the  reported  percentages  of  rubber  may  be  too  low. 
It  is  to  be  noted,  in  comparing  the  results  with  those  given  by  rubber 
chemists,  that  the  latter  often  report  the  percentage  of  crude  rubber 
extracted  by  some  process  that  carries  out  with  it  a  larger  or  smaller 
amount  of  impurities,  these  sometimes  constituting  as  high  as  30  per 
cent  of  the  yield.  Even  by  the  method  used  in  the  present  investi- 
gation, a  varying  amount  of  impurity  doubtless  finds  its  way  through 
to  the  benzene  extract,  so  that  the  result  is  not  a  pure  hydrocarbon. 
However,  the  percentages  reported  are  believed  to]approximate  those 
obtained  by  commercial  processes,  and  in  any  event  they  may  be 
used  for  purposes  of  comparison  between  the  species  and  forms. 

Duplicate  analyses  were  run  of  most  samples,  and  when  unusually 
high  results  were  obtained  they  were  checked  by  a  larger  series  of 
examinations.  The  close  agreement  in  the  results  obtained  adds  to 
the  confidence  in  the  method  employed. 


THE   MOST   IMPORTANT   SPECIES.  15 

V.  THE  MOST  IMPORTANT  SPECIES. 

Of  the  64  species  examined  and  found  to  contain  rubber,  there  are 
16  which  seem  sufficiently  promising  to  warrant  treatment  in  some 
detail.  These  have  been  selected  because  of  their  comparatively  high 
rubber-content,  this  running  from  1  to  8  per  cent  of  the  dry  weight,  at 
least  for  mature  leaves.  Whether  or  not  this  is  sufficient  to  encourage 
experimental  planting  with  a  view  to  growing  the  rubber  commercially 
is  a  matter  concerning  which  the  writers  are  not  competent  to  judge. 
It  is  certain,  however,  that  such  percentages  have  not  been  approxi- 
mated in  any  latex-bearing  plant  native  to  the  United  States  that  has 
been  heretofore  examined,  and  since  this  fact  may  render  them  of 
special  interest  to  the  rubber  industry  as  well  as  to  the  botanist,  the 
following  descriptive  accounts  have  been  prepared. 

THE  GENUS  ASCLEPIAS,  OB  TRUE  MILKWEEDS. 

Since  most  of  the  species  here  discussed  belong  to  the  genus  Asclepias, 
a  preliminary  account  of  this  group  of  plants  will  avoid  repetition. 
The  genus  comprises  somewhat  more  than  100  species,  and  is  best 
represented  in  tropical  and  southern  Africa.  About  46  species  are 
native  to  the  United  States  and  Canada,  and  a  number  of  these  are 
represented  by  additional  varieties  and  forms  of  more  or  less  importance. 
All  of  the  North  American  species  are  perennials,  with  deep  roots, 
which  spread,  in  most  cases  at  least,  by  horizontal  branches  which 
give  rise  at  intervals  to  vertical  roots,  these  in  turn  dividing  when 
they  reach  the  surface  of  the  soil  to  form  the  crown  from  which  new 
stems  arise.  In  some  species  this  creeping  of  the  roots  is  so  extensive 
that  a  single  plant  comes  to  occupy  areas  of  large  extent,  covering 
them  with  dense  stands  of  leafy  stems.  Vegetative  propagation  by 
portions  of  the  horizontal  roots  is  entirely  feasible.  The  horizontal 
root-branches  have  been  almost  universally,  mistaken  for  rootstocks, 
but  their  true  nature  has  been  recently  pointed  out  by  Eggleston  (in 
Marsh  et  al,  1920).  But  in  at  least  one  species  (A.  subulata)  there 
seems  to  be  only  a  single  taproot,  without  connection  with  other  tap- 
roots, but  this  needs  verification. 

A  few  of  the  milkweeds  have  woody  stems,  but  those  here  described 
die  down  to  near  the  base  each  autumn  or  winter,  new  shoots  appear- 
ing from  the  same  root  or  stump  at  the  beginning  of  the  following 
growing-season,  which  usually  follows  the  melting  of  the  snows  in 
the  colder  regions  or  the  first  rains  in  the  warmer  and  arid  south- 
western districts.  When  the  stems  are  injured  new  ones  spring  up 
to  take  their  place.  There  can  therefore  be  no  doubt  that  if  a  crop 
is  once  taken  off  for  any  purpose  it  will  be  replaced  by  a  new  one, 
either  in  the  same  or  in  the  following  season.  This  has  been  experi- 
mentally demonstrated  in  the  case  of  several  of  the  species.  Three 
''crops"  of  shoots  of  Asclepias  mexicana  have  been  harvested  in  one 


16  RUBBER-CONTENT   OF  NORTH   AMERICAN   PLANTS. 

season.  The  stalks  are  erect  in  some  species,  spreading  in  others,  and 
a  few  inches  to  8  feet  or  more  long,  but  they  are  always  straight  and  with 
few  or  no  branches.  The  bast  fibers  are  long  and  stronger  than  in  most 
native  plants,  for  which  reason  they  have  been  examined  to  some  extent 
with  a  view  to  their  utilization  in  the  manufacture  of  cordage  and  cloth. 

The  leaves  are  nearly  always  opposite  or  whorled  and  occur  at 
regular  intervals  to  the  top,  the  upper  leaves  being  nearly  as  large  as 
the  lower  ones.  It  is  not  unusual,  however,  for  the  lowest  and  the 
topmost  leaves  to  be  alternate  on  the  stem,  and  in  a  few  cases  all  of 
the  leaves  are  greatly  reduced  in  size.  In  A.  subulata  this  reduction 
is  carried  to  so  great  an  extent  that  for  the  greater  part  of  the  year  the 
plant  consists  only  of  naked,  rush-like  stems.  Many  of  the  species 
are  clothed  with  a  woolly  tomentum  of  plant-hairs,  while  others  are 
quite  smooth  and  naked. 

The  flowers  appear  in  clusters,  technically  called  umbels,  in  the 
axils  of  the  upper  leaves  and  often  at  the  apex  of  the  stem.  Their 
structure  is  exceedingly  complicated  as  the  result  of  adaptation  to 
insect  pollination.  The  principal  pollinators  are  bees,  wasps,  ants, 
and  flies  of  various  species.  It  is  quite  certain  that  any  species  of 
milkweed  would  fail  to  set  seed  if  grown  where  the  particular  insect 
necessary  for  its  pollination  is  absent.  The  flowers,  at  least  of  some 
species,  produce  an  abundance  of  nectar,  and  bee-keepers  report  an 
average  production  of  50  pounds  of  honey  of  good  quality  per  colony 
of  bees  year  after  year  from  some  species,  especially  A.  syriaca. 

The  flowers  are  followed  by  pods,  and  each  pod  is  filled  with  numer- 
ous seeds.  The  seeds  bear  tufts  of  soft,  downy  hairs  at  the  apex, 
this  down  being  especially  noticeable  as  the  pod  opens,  when  the  hairs 
may  be  from  1  to  nearly  2  inches  long.  Natural  propagation  is  chiefly 
by  seeding.  Germination  tests  of  two  collections  of  seed  of  Asclepias 
mexicana  showed  that  60  per  cent  were  viable;  one  collection  of  A. 
verticillata  gave  64  per  cent,  one  of  A.  sullivanti  gave  16  per  cent,  and 
in  one  collection  of  A.  syriaca  no  germination  was  effected.  The  seeds 
are  slow  to  germinate,  and  some  difficulty  has  been  experienced  in 
getting  field-sown  seeds  to  grow  at  all.  More  favorable  results  are  to  be 
expected  from  seed  gathered  at  time  of  full  maturity.  Alternating  tem- 
peratures seem  to  be  necessary  for  successful  germination  in  some  cases. 

Several  of  the  species,  especially  those  of  the  narrow-leaved  group, 
are  poisonous  to  stock,  as  has  been  demonstrated  by  Hall  and  Yates 
(1915)  and  by  Marsh  et  al  (1920).  On  the  other  hand,  the  young 
shoots  and  young  pods,  at  least  of  A.  syriaca,  are  cooked  and  eaten 
with  impunity. 

The  genus  is  characterized  by  the  presence  of  a  milky  sap  or  latex, 
which  is  carried  in  special  vessels  of  the  laticiferous  tissue.  As  in  the 
case  of  Hevea,  Ficus,  etc.,  the  rubber  is  borne  in  the  latex,  but  the 
plants  are  so  small  that  extraction  by  a  process  of  tapping,  such  as  is 


THE   MOST   IMPORTANT    SPECIES.  17 

practiced  on  those  trees,  is  here  out  of  the  question.  A  more  rational 
method  of  preparation  would  consist  in  cutting  the  plants  near  the 
ground,  after  which  they  would  be  passed  between  rollers  and  the 
rubber  extracted  by  chemical  or  mechanical  means  with  the  aid  of 
water.  It  is  possible  that,  by  using  properly  constructed  machinery, 
the  fiber  as  it  comes  from  the  mill  could  be  processed  and  utilized  in 
manufacturing  low-grade  fabrics,  or  if  this  is  not  feasible,  the  crushed 
residue  would  furnish  material  for  the  manufacture  of  paper-pulp. 

ASCLEPIAS   SUBULATA    (DESERT  MlLKWEED). 

Description. — Plant  a  rounded  perennial  herb,  woody  at  base  in 
some  types,  3  to  8  feet  high,  2  to  6  or  even  10  feet  broad  when  growing 
in  low  places  where  water  accumulates  after  rains,  narrow  and  few- 
stemmed  when  on  dry  upland  slopes  (this  upland  form  is  more  woody 
and  may  be  a  distinct  race);  roots  deep,  ending  above  in  a  much- 
branched  crown;  stems  usually  12  to  30  in  poor  plants,  but  up  to  500 
or  more  in  robust  forms,  straight,  either  simple  or  with  a  few  straight 
branches,  glabrous  but  with  a  gray  bloom;  leaves  usually  reduced  to 
linear,  very  acute  scales  3  inches  or  less  long,  but  more  developed  in 
good  seasons,  and  then  often  quite  copious;  flowers  appearing  after  the 
rains  regardless  of  season,  dull  greenish  white,  in  rounded  clusters 
which  terminate  erect  or  ascending  branches  of  the  inflorescence;  pods 
from  erect  to  pendent  on  curved  stalks,  fusiform,  very  slenderly  taper- 
ing to  the  apex,  3.5  to  6  inches  long,  0.5  to  0.75  inch  thick,  smooth  and 
glabrous  or  only  minutely  puberulent. 

References.— Torrey,  Pacif.  R.  Rept.,  5:  362,  pi.  7,  1857.     Gray,  Syn.  Fl.,  21 :96,  1878. 

Distribution  and  ecology. — This  is  preeminently  a  desert  species, 
although  it  ranges  from  southeastern  California,  western  Arizona,  and 
western  Sonora  across  Lower  California  to  the  islands  off  the  west 
coast.  The  northernmost  known  station  is  Searchlight,  Nevada, 
while  to  the  east  it  extends  but  slightly  beyond  Florence,  in  south- 
western Arizona.  To  the  south  it  grows  at  least  as  far  as  Cape  San 
Lucas  and  Guaymas.  At  the  latter  place  it  is  common  and  said  to  be 
known  as  yumete.  The  only  known  station  in  upper  California, 
outside  of  the  desert  area,  is  Tecate  Valley,  near  the  Mexican  border, 
southeast  of  San  Diego.  Even  within  the  area  as  thus  delimited,  A. 
subulata  nowhere  occurs  in  great  abundance.  Usually  it  grows  as 
scattered  clumps  on  the  foothill  slopes  and  in  dry,  stony  streamways, 
which  are  flooded  with  storm-water  for  a  short  period  after  the  infre- 
quent rains.  Occasionally  the  plants  are  found  growing  in  better  soil 
of  depressions  where  the  water  remains  for  some  time.  Here  they 
readily  respond  to  the  improved  conditions  and  form  large,  bushy 
plants,  with  a  greatly  increased  number  of  stems.  For  example,  at 
Sentinel,  Arizona,  a  few  plants  were  found  in  such  depressions  which 


18  RUBBER-CONTENT   OF   NORTH   AMERICAN   PLANTS. 

weighed  from  12  to  16  pounds,  and  H.  R.  Reed  has  collected  at  Dome, 
Arizona,  plants  with  over  500  stems  and  weighing  25  to  30  pounds. 
The  largest  plants  thus  far  located  were  discovered  by  Dr.  F.  E. 
Clements  at  a  point  about  15  miles  east  of  Mesa,  Arizona.  One  of 
these  measured  6  feet  high,  7  feet  broad,  and  had  approximately  2,000 
ultimate  branches.  It  was  estimated  to  weigh  close  to  100  pounds. 

The  difference  just  mentioned  in  regard  to  habit  raises  the  question 
as  to  the  possible  presence  of  two  distinct  races  or  subspecies.  There 
is  some  evidence  to  indicate  that  this  is  the  case.  The  woody  type, 
as  one  form  may  be  called,  grows  mostly  on  the  bench-lands  and 
stony  slopes,  sometimes  also  in  dry  streamways.  It  is  found  as 
scattered  individuals  and  withstands  excessive  drought,  as  shown,  for 
example,  by  its  presence  near  the  very  dry  and  well-drained  summit  of 
Picacho  Peak,  California,  at  an  altitude  of  1,945  feet.  The  plants  of 
this  form  are  almost  shrubby,  the  lower  portion  of  the  stems  being 
decidedly  woody.  The  branches  are  comparatively  few,  commonly 
about  30  or  40  at  midway  from  the  ground,  and  are  not  widely  spread- 
i  ng.  Individuals  as  much  as  8  feet  high  have  been  noted  on  Picacho 
Peak  by  Mr.  Reed.  Perhaps  this  form  is  to  be  identified  as  A.  alhi- 
cans  Watson  (Proc.  Am.  Acad.  Sci.  24:  59,  1889). 

The  second  form,  which  may  be  spoken  of  as  the  stooling  type,  has 
been  found  only  in  dry  streamways,  or  in  depressions,  or  on  plains 
where  apparently  there  is  a  little  more  moisture  than  at  places  where 
the  woody  type  occurs.  Yet  even  in  these  more  favored  spots  the 
conditions  are  decidedly  arid  and  the  plants  are  fully  exposed  to  desert 
conditions.  These  statements  apply  only  to  plants  of  Arizona  and 
southern  California,  since  the  conditions  under  which  they  grow  in 
Mexico  are  not  well  understood.  The  plants  of  this  type  are  woody 
only  at  the  very  base,  which  branches  or  stools  near  the  ground  into 
a  large  number  of  erect  or  ascending  reedy  stems.  A  specimen  is 
illustrated  in  plate  1.  The  number  of  stems  varies  from  about  100  to 
600  or  more,  and  since  they  average  fully  as  long  as  in  the  other  type, 
the  total  weight  of  the  plant  is  usually  much  greater.  Small  plants 
will  weigh  from  5  to  15  pounds;  exceptionally  large  ones  are  estimated 
to  weigh  50  to  100  pounds. 

In  comparing  the  two  forms  of  Asclepias  subulata  as  to  their  relative 
value  for  agricultural  purposes,  it  is  first  noted  that  the  stooling  type 
will  yield  much  the  greater  tonnage,  that  its  growth  is  more  rapid, 
and  that  it  could  be  more  easily  harvested  by  mowing  machines.  On 
the  other  hand,  the  woody  type  is  more  drought-resistant  and  carries 
a  lower  percentage  of  moisture.  The  moisture-content  of  two  woody 
plants,  as  determined  by  Mr.  Reed,  was  43  and  49  per  cent,  respectively, 
while  one  plant  of  the  stooling  type  contained  60  per  cent.  The 
samples  were  taken  8  days  apart  and  from  different  localities,  which 
may  account  for  the  difference,  but  this  ratio  is  about  what  one 


THE   MOST   IMPORTANT    SPECIES.  19 

would  expect  from  a  casual  examination  of  average  stems  and  may 
be  accepted  for  purposes  of  general  comparison.  Only  two  samples 
of  the  woody  type  have  been  analyzed.  These  carried  less  rubber  than 
the  average  for  the  stooling  type,  but  the  results  might  be  quite 
different  if  a  large  series  could  be  examined.  If  the  differences  in 
size  and  habit  as  above  noted  are  purely  ecological,  it  may  be  assumed 
that  only  the  stooling  type  would  be  obtained  under  cultivation, 
regardless  of  the  source  of  the  seeds.  If,  however,  these  types  repre- 
sent hereditary  races,  then  the  selection  of  seed  becomes  a  matter  of 
importance.  In  this  latter  case  attempts  should  be  made  to  hybridize 
the  forms  in  order  to  obtain  a  progeny  combining  the  best  characters 
and  with  as  high  a  rubber-content  as  possible.  Judging  from  experi- 
ence with  other  plant  species,  it  is  quite  probable  that  the  percentage 
of  rubber  can  be  considerably  increased  through  hybridization  and 
selection,  and  if  each  of  the  types  already  distinguished  is  itself  com- 
posed of  minor  strains,  as  is  likely  to  be  the  case,  these  should  first 
be  segregated  in  order  to  furnish  material  as  a  basis  for  the  work. 
An  increase  in  the  rubber-content  is  one  of  the  requirements  necessary 
to  bring  such  plants  into  the  list  of  agricultural  possibilities. 

The  desert  habitat  of  Asckpias  subulata  indicates  that  it  may  be 
brought  under  cultivation  more  cheaply  than  other  species.  It  cer- 
tainly could  be  grown  on  land  that  is  now 'considered  worthless  because 
of  the  lack  of  water  and  which  could  therefore  be  had  at  a  minimum 
price.  The  low  water  requirement  is  evidenced  by  the  growth  made 
under  desert  conditions.  The  normal  rainfall  at  Sentinel,  Arizona, 
whence  came  many  of  the  samples,  is  4.2  inches;  that  at  Yuma  is  3.1 
inches.  The  maximum  temperature  for  1919  at  Sentinel  was  120°  F. 
and  the  minimum  22°  F. 

The  better  type  of  the  desert  milkweed  grows  in  dry  streamways 
and  in  slight  depressions  on  the  desert  mesas,  indicating  the  desir- 
ability of  increasing  the  soil-moisture,  either  by  slight  irrigation  or 
through  cultivation  and  other  practices  of  dry  farming.  A  system  of 
cross-furrowing  or  checking  that  would  retard  the  run-off  after  rains, 
together  with  surface  tillage,  would  doubtless  increase  the  tonnage. 
Under  such  methods,  and  especially  under  irrigation,  the  plants  would 
probably  develop  normal  foliage,  but  what  effect  this,  or,  for  that 
matter,  the  methods  themselves,  would  have  upon  the  total  yield  of 
rubber  can  be  determined  only  by  experiment. 

It  is  highly  desirable  that  cheap  and  effective  methods  of  vegetative 
reproduction  be  found  for  perennial  plants  to  be  grown  for  then*  rubber. 
Since  Asckpias  subulata  grows  from  a  taproot  and  is  apparently  lacking 
in  the  horizontal,  connecting  root  branches  of  most  milkweeds,  it 
seems  to  propagate  itself  in  nature  only  by  seeds.  However,  some 
recent  and  incomplete  experiments  made  by  Mr.  H.  R.  Reed,  while 
connected  with  the  Experimental  Farm  of  the  United  States  Depart- 
ment of  Agriculture  at  Bard,  California,  tend  to  show  that  vegetative 


20  RUBBER-CONTENT  OF  NORTH  AMERICAN   PLANTS. 

reproduction  can  be  practiced  without  great  difficulty.  This  investi- 
gator worked  entirely  with  the  stooling  type,  and,  in  addition  to  mak- 
ing a  number  of  successful  transplants,  also  grew  plants  from  portions 
of  the  crown  removed  for  this  purpose.  The  division  was  made  in 
such  a  manner  as  to  leave  a  few  coarse,  fibrous  roots  attached  to  the 
propagules,  which  were  grown  first  in  a  pot  and  then  in  the  open  ground. 
The  divisions  were  made  January  19,  1921.  By  March  26  the  young 
shoots  were  4  inches  high.  During  the  same  period  the  parent  plant, 
which  had  been  pruned  back  and  transferred  to  the  experiment  station 
at  Bard,  California,  along  with  the  propagules,  had  sent  up  new 
shoots  6  inches  long.  Other  portions  of  the  same  crown  were  started 
without  irrigation  by  covering  with  2  feet  of  sandy  soil,  but  were  lost 
when  shallowly  planted  in  the  field.  This  was  probably  due  to  the 
extreme  exposure  to  the  hot  sun  before  the  new  roots  had  become 
established. 

In  order  to  determine  whether  plants  would  send  up  new  shoots  if 
the  tops  were  removed,  Mr.  Reed  cut  back  some  bushes  on  January 
30,  1921.  These  were  growing  near  Dome,  Arizona.  One  of  the 
plants,  which  had  96  stems  when  cut  back,  had  sent  up  260  new 
shoots  by  May  25  of  the  same  year.  At  this  time  the  new  growth 
was  30  inches  high  and  "absolutely  full"  of  latex. 

The  above  experiments  indicate  (1)  that  Asclepias  subulata  can  be 
successfully  transplanted;  (2)  that  when  plants  are  cut  back  a  new 
"crop"  of  stems  will  be  produced;  and  (3)  the  possibility  of  vegeta- 
tive reproduction  by  division  of  the  crowns. 

Rubber-content. — It  will  be  noted  from  table  1  that  all  of  the  analy- 
ses are  of  the  stems.  This  is  due  to  the  fact  that  the  leaves  are  so 
small  and  often  so  sparse  at  tune  of  collection  as  to  be  negligible.  The 
results  show  that  the  laticiferous  tissue  of  the  stem  takes  over  the 
rubber-holding  properties  of  the  leaves  of  other  species,  just  as  it  also 
assumes  their  photosynthetic  activities.  The  green  and  glabrous 
nature  of  the  stem  is  doubtless  correlated  with  this  modified  role. 

The  tabulated  analyses  give  evidence  of  a  considerable  fluctuation 
in  the  percentage  of  rubber  present.  The  possible  explanations  of 
this  include  seasonal  variation,  the  effect  of  different  environments, 
the  presence  of  genetic  strains,  and  a  lack  of  uniformity  in  the  selec- 
tion of  the  material  to  be  analyzed.  Only  by  detailed  experiments 
can  the  actual  causes  be  determined.  It  is  noted,  however,  that 
almost  uniformly  low  yields  were  obtained  from  the  samples  gathered 
at  Dome,  Arizona,  on  January  30, 1921.  It  is  possible  that  the  plants 
at  this  station  are  of  a  poor  type,  but  the  two  collected  on  October  18, 
1920  (Nos.  1121  and  1122),  at  the  .same  place  ran  fairly  high.  The 
explanation  is  suggested  that  all  plants  are  higher  in  rubber-content 
during  the  autumn  than  in  the  early  spring.  This  seems  not  unlikely, 
since  the  stems  dry  up  and  become  brittle  during  the  winter  months, 
thus  rendering  probable  the  decomposition  of  rubber  at  this  time. 


THE   MOST    IMPORTANT   SPECIES. 


21 


These  dry  stems  are  not  unlike  the  woody  base,  which  always  carries 
less  rubber  than  the  green  middle  portions. 

TABLE  1. — Chemical  analyses  of  Asclepias  subulata.1 


No. 

Place  of  collection. 

Date  of 
collection 

Part  of  plant 
analyzed. 

Acetone 
extract. 

Rubber 
(beneene 
extract). 

892 

Near  Yuma,  Ariz  

Oct.    .  . 

Stems  

p.  ct. 
14  0 

p.  ct. 
5  2 

954 

Sentinel  Ariz.  *  

Sept.  29 

Stems  

5  8 

2  l 

Base  

4  8 

0  5 

955 

do     

...do.... 

Stems  

7  3 

2  6 

956 

do  

...do.... 

Stems  

9  3 

4  4 

957 

do  

...do  

Stems  without  bark  

6  7 

1  7 

Stem-bark  

16  5 

5  3 

958 

do                 

...do.    .. 

Stems     

8  8 

3  5 

971 

do                   

May 

Young  stems  

9  9 

0  8 

Old  stems  

10  1 

2  5 

997 

Imperial  Valley,  Calif  .... 

May    5 

Stems  

14  1 

3  3 

1011 

Sentinel  Ariz.              .... 

Apr.     3 

Young  stems  

9  8 

1  8 

Old  stems  

10  2 

2  5 

1015 

do  

Sept.  29 

Young  stems  

14  6 

6  5 

W^oody  base        

4  7 

1  3 

1120 

Tinajas  Alias  Ariz.  '.,... 

Mar.  19 

Upper  half  

16  6 

5  4 

Lower  half  

15  2 

2  1 

1121 

Oct.    18 

8-inch  tips  

14  4 

5  4 

Next  12  inches  

14  8 

2  5 

Lower  part  

9  4 

1  9 

Base  

8  6 

0  8 

1122 

do  

Oct.   18 

Entire  stem  

11  9 

3  9 

1123 

Near  Picacho  Calif  

Dec.     5 

Tops  

2  5 

Middle   

2  2 

Base   

2  0 

1125 

Sept.  29 

Tips  

18  5 

3  9 

Next  below  

16  5 

5  7 

Lower  part  

12  3 

3  8 

Base  

11  1 

3  0 

1126 

do  

Sept.  29 

Top  

16  0 

4  5 

Middle  

2  8 

Base             

10  4 

1  3 

1127 

Yuma  Mesa  Ariz 

Jan.    19 

Top  

14  6 

4  5 

Middle  

16  9 

6  4 

Base     

14  1 

4  3 

B7 

Near  Dome  Ariz. 

Jan.   30 

Top  

16  4 

3  9 

Middle  

16  0 

3  4 

Base  

11  5 

2  4 

B8 

do 

do 

Upper  half  

16  7 

2  3 

Lower  half       

13  5 

1  2 

B9 

....do  

...do.... 

Top  
Middle  

18.4 
16  2 

3.1 
2  5 

Base           

15  4 

1  9 

BIO 

do 

do 

Upper  half  

17  5 

1  8 

Lower  half  

17.4 

2  2 

Bll 

do  

...do.... 

Top  
Middle  

18.6 
15  2 

2.5 
1  6 

Base  

10  9 

1.0 

1  In  this  and  in  all  following  tables  the  acetone  extract  includes  the  resins  and  other  acetone- 
Boluble  substances,  whereas  the  benzene  extract  is  taken  to  represent  the  amount  of  crude  rubber, 
although  a  negligible  amount  may  have  been  extracted  with  the  acetone  (see  p.  14).     With 
few  exceptions  the  percentages  are  based  on  absolutely  dry  weight  of  the  plant  parts  analyzed. 

2  Growing  in  depression.       3  From  granite  slope.       *  Estimated  weight,  27  pounds,  green. 


22  RUBBER-CONTENT  OF  NORTH  AMERICAN  PLANTS. 

The  several  series  of  analyses  of  different  parts  of  the  same  plant 
indicate  conclusively  that  the  percentage  increases  up  the  stem  until 
the  young  tips  are  reached,  when  it  again  falls  off  in  some  cases.  The 
continued  increase,  even  to  the  uppermost  sections,  as  reported  under 
a  few  numbers,  may  be  due  to  a  different  selection  of  the  height  at  which 
the  cuts  were  made.  For  a  comparison  between  different  plants  it  would 
be  better  to  take  equal  portions  of  all  parts  above  the  woody  base. 

ASCLEPIAS   SULLIVANTI. 

Description. — Plant  a  stout,  strictly  erect  perennial  herb,  2  to  5 
feet  high;  roots  connected  by  underground  branches,  which  give  rise 
to  clumps  of  stems  at  irregular  intervals,  but  1  to  3  feet  are  common 
distances  between  the  clumps;  stems  sometimes  solitary,  but  usually 
2  to  6  from  the  base,  simple  or  with  few  branches,  straight,  very  smooth 
and  without  hairs;  leaves  numerous  to  the  top,  7  to  12  pairs  on  each 
stem,  sometimes  3  or  4  in  a  whorl  at  some  of  the  joints,  oblong  or  some- 
what ovate,  4  to  6  inches  long,  1.5  to  3  inches  wide,  thick,  smooth,  not 
hairy;  flowers  purplish  or  flesh-colored,  appearing  from  the  middle  of 
June  to  early  September,  many  hi  a  spherical  stalked  cluster  (umbel) 
which  terminates  the  stem,  or  the  clusters  sometimes  also  in  the  axils 
of  the  upper  leaves;  pods  on  curved  stalks,  erect,  ovoid,  3  to  5  inches 
long,  about  1  inch  thick,  not  hairy,  but  usually  with  small  blunt  proc- 
esses near  the  apex,  maturing  in  late  August  and  September. 

References.— Gray,  Syn.  FL,  21:91,  1878.  Britton  and  Brown,  111.  Fl.,  ed.  2:27,  fig 
3390,  1913. 

Distribution  and  ecology. — The  range  of  this  species  is  from  southern 
Ontario  and  Ohio  to  Kansas,  Nebraska,  and  Minnesota.  While  it 
may  be  expected  almost  anywhere  throughout  this  section  where  soil 
and  moisture  conditions  are  suitable,  there  are  but  few  records  of  its 
occurrence  in  abundance.  Professor  T.  J.  Fitzpatrick,  of  the  Univer- 
sity of  Nebraska,  reports  it  as  common  on  many  of  the  bottom  lands 
along  streams  in  southern  Iowa,  and  especially  along  the  Chariton 
River  opposite  Centerville.  Other  localities  where  it  may  be  obtained 
include  Lincoln,  Nebraska;  Manhattan,  Kansas;  Baltimore  and  Ohio 
Railway  near  Kimball,  Erie  County,  Ohio. 

Asclepias  sullivanti  grows  only  on  low  land  where  the  soil  is  moist. 
At  the  present  time  it  seems  to  be  most  abundant  along  streams  and 
lake-shores,  where  the  land  is  overflowed  at  certain  seasons,  but  this 
must  not  be  taken  as  evidence  that  such  conditions  are  essential  to  its 
growth.  Possibly  it  was  once  much  more  common  on  better  drained 
and  drier  soil,  from  which  it  has  been  crowded  by  agricultural  practice. 
Its  distribution  indicates  that  it  can  endure  very  low  temperatures  in 
winter,  at  least  as  low  as  30°  F.  below  zero,  and  that  a  fairly  high  summer 
temperature  and  humidity  are  at  least  suitable  and  perhaps  requisite 
to  its  best  development. 


THE   MOST   IMPORTANT   SPECIES. 


23 


Since  this  is  a  species  of  rich,  moist  bottom-lands,  it  is  probably  not 
suited  to  arid  conditions.  If  this  assumption  is  correct,  Asclepias 
sullivanti  would  seem  to  hold  but  little  promise  of  success  as  a  cultivated 
plant,  since,  regardless  of  the  amount  of  rubber  it  might  yield,  other 
crops  could  probably  be  grown  at  a  greater  profit.  However,  the  adapt- 
ability of  the  species  to  other  soils  and  the  variation  in  rubber-content, 
which  would  undoubtedly  accompany  such  a  change  of  habitat,  are 
matters  which  should  be  tested  experimentally,  for  this  is  in  other 
respects  one  of  the  most  promising  of  all  of  the  milkweeds  thus  far 
examined.  The  tall-growing  erect  habit  of  A.  sullivanti  is  much  in  its 

TABLE  2. — Chemical  analyses  of  Asclepias  sullivanti. 


No. 

Place  of  collection. 

Date  of 
collection. 

Part  of  plant 
analyzed. 

Acetone 
extract. 

Rubber 
(benzene 
extract). 

387 

Aug.    9 

p.  ct. 
8.7 

p.  ct. 
5  1 

Stems     

6  2 

0  4 

817 

Ellsworth   Kans  ' 

Sept.    . 

Leaves       

5  0 

Stems  

8  2 

886 

Aug     2 

Leaves         

9  5 

8  1 

Stems     

6.0 

0  2 

1016 

do 

Aug.  28 

9.4 

5  4 

Stems  

6.3 

0  5 

1017 

do  ' 

do 

Leaves       

8  5 

3  0 

Stems     

6.7 

0  2 

1018 

do.1  

do  

Leaves  

8.5 

1  2 

Stems     

7.8 

0  9 

1019 

do.  •    

..   do  

7.1 

3  0 

Stems     

7.0 

0  9 

1020 

do  *       

.  .   do 

8.1 

3  6 

4.7 

0  4 

1021 

do.4  

...do  

Leaves   

7.7 

4  0 

Stems     

7.1 

0  2 

1022 

do.  *    

Sept.    6 

Leaves       

7.3 

4  1 

Stems           

6.4 

0  2 

1023 

do       

.  .  do  ... 

Leaves*       

10.6 

2  7 

Stems     

9.3 

0.5 

1024 

do.  •  

...do  

8.2 

2  0 

Stems          

1.2 

0  3 

1025 

do.  *      

Sept.  14 

Leaves           

10  9 

2  7 

6.4 

0.4 

1026 

do.7  

...do  

9.9 

3  6 

2.3 

0  3 

1027 

do.«  

...do  

11.7 

4  2 

6.1 

0  2 

1028 

do.  *  

Sept.  22 

Leaves       

7.9 

3  0 

4.3 

0  2 

1028 

do.7  

...do  

10.0 

4  2 

4.6 

0  6 

1030 

do.*  

...do  

8.1 

4  2 

Stems       

1.9 

0  1 

1031 

do  

Sept.  12 

Leaves*   

7.9 

2  3 

1  Plants  injured  by  frost  and  leaves  with 

more  blackish  patches. 
1  Verified  by  6  duplicate  analyses. 
1  Same  locality  as  1016. 


4  From  5  plants. 

*  Five  plants  from  same  spot  as  1020. 
1  Five  plants  from  same  spot  as  1021. 
7  Five  plants  from  same  spot  as  1023. 


24  RUBBER-CONTENT   OF   NORTH   AMERICAN  PLANTS. 

favor,  since  this  is  conducive  to  ease  of  manipulation  in  harvesting. 
The  robust  stems,  even  of  wild  plants,  are  sometimes  0.75  inch  thick, 
but  they  are  not  so  firm  as  to  render  harvesting  by  machinery  especially 
difficult.  The  height  of  the  stems,  well-clothed  to  the  summit  with 
leaves  of  fairly  large  size,  indicate  that  the  tonnage  that  could  be 
grown  per  acre  is  probably  greater  than  in  any  other  species,  but  unless 
forms  can  be  found  in  which  the  stems  carry  a  larger  percentage  than 
the  chemical  analyses  usually  show,  it  will  be  necessary  to  use  only  the 
leaves,  which  constitute  but  50  per  cent  of  the  total  weight  of  the  plant. 

Eubbei-content. — This  milkweed  has  yielded  the  highest  percentage 
of  rubber  of  all  the  species  thus  far  tested.  The  most  remarkable 
records  are  those  of  plant  No.  817,  with  5.0  per  cent  in  the  leaves  and 
8.2  per  cent  in  the  stem;  and  No.  886,  with  8.1  per  cent  in  the  leaves 
and  only  a  trace  hi  the  stem.  The  high  percentage  in  the  stem  of  No. 
817  is  unique  and  needs  verification  through  the  examination  of  a 
larger  series  of  plants.  Yet  the  results  seem  trustworthy,  since  they 
were  checked  by  a  second  analysis,  and  both  analyses  were  carried 
out  with  the  greatest  care  and  in  the  same  manner  as  the  others.  The 
average  of  the  19  lots  of  leaves  analyzed  is  3.7  per  cent,  and  there  is 
the  unaccountable  individual  variation  always  encountered  when  a 
large  series  of  plants  is  analyzed.  The  encouraging  feature  is  the 
remarkably  high  percentage  in  a  few  cases,  possibly  indicating  superior 
strains  suitable  as  a  beginning  in  breeding  experiments. 

ASCLEPIAS   SYRIACA   (COMMON   MlLKWEED). 

Synonym:  Asclepias  cornuti. 

Description. — Plant  a  stout,  erect  perennial  herb,  3  to  6  and  some- 
times 7  feet  high;  roots  probably  spreading  bycleep  horizontal  branches; 
stems  usually  several  at  a  place,  often  very  numerous  and  crowded  so 
as  to  form  thickets  of  large  extent,  simple  up  to  the  flower-clusters, 
straight,  at  first  finely  soft-hairy;  leaves  numerous  to  the  top,  usually 
20  or  more  on  each  stem,  opposite,  lance-oblong  or  broadly  elliptic, 
5  to  8  inches  long,  2  to  4  or  rarely  6  inches  wide,  thick,  pale,  soon 
smooth  and  glabrous  above,  but  minutely  downy  beneath;  flowers 
dull  purple  to  white,  appearing  from  late  June  to  September,  many  in 
stalked  umbels  from  the  upper  leaf-axils;  pods  on  deflexed  stalks,  erect, 
ovoid  and  acuminate,  3  to  5  inches  long,  1  to  1.25  inches  thick,  gray 
with  a  dense  wool-like  tomentum,  with  small  soft  spine-like  projec- 
tions, maturing  in  late  August  and  September. 

References.— Gray,  Syn.  Fl.,  21:91,  1878  (as  A.  cornuti).  Britton  and  Brown,  111.  Fl., 
ed.  2:30,  fig.  3398,  1913. 

Distribution  and  ecology. — This  is  the  most  abundant  milkweed  in 
the  eastern  United  States  and  Canada,  where  it  grows  from  New 
Brunswick  to  North  Carolina  and  west  as  far  as  Kansas  and  Saskatch- 


THE   MOST   IMPORTANT   SPECIES.  25 

ewan.  It  is  a  species  especially  of  fields  and  waste  places,  commonly 
coming  in  where  the  soil  has  been  disturbed.  Railroad  rights-of-way 
are  therefore  often  lined  with  it.  The  climate  of  southern  Michigan 
is  apparently  well  suited  to  the  growth  of  the  plants,  since  they  are 
reported  as  very  abundant  in  that  region.  Specific  localities  whence 
samples  may  be  obtained  include  London,  Ontario;  Stevensville, 
Michigan;  and  Lincoln  and  Madison,  Nebraska.  The  plant  has  been 
successfully  grown  near  Akron,  Ohio,  where  it  was  used  experimentally 
in  the  preparation  of  rubber  by  Fox  (1910). 

Although  Asclepias  syriaca  is  usually  reported  as  growing  in  waste 
places,  there  is  no  direct  evidence  available  that  it  will  grow  on  soils 
unsuited  to  any  other  crop.  It  will  make  a  better  growth  than  most 
agricultural  plants  on  shallow  soil  and  in  worn-out  pastures  and  will 
flourish  in  places  so  stony  that  cultivation  is  impossible.  But  since 
such  conditions  seldom  prevail  over  large  areas,  the  competition  with 
other  crops,  such  as  beans  and  corn,  will  need  to  be  taken  into  account. 
This  species  belongs  to  the  more  humid  eastern  climate  and  is  prob- 
ably quite  unsuited  to  the  arid  districts  of  the  West  and  Southwest. 
Its  ability  to  grow  and  form  large  plants  in  the  climate  of  New  Bruns- 
wick and  Saskatchewan  indicates  that  it  is  the  most  hardy  of  the 
American  milkweeds  and  one  of  the  few  to  be  considered  for  Canada 
or  the  States  along  the  Canadian  border.  Other  characteristics 
favorable  to  this  species  include  its  large  size,  robust  habit,  abundance 
of  foliage,  and  especially  its  ability  to  spread  rapidly  by  underground 
parts,  thus  coming  to  cover  large  areas  with  a  solid  stand  of  erect, 
leafy  stems. 

Rubber-content. — The  first  American  milkweed  to  be  tested  for  its 
rubber  was  Asclepias  syriaca,  and  up  to  the  time  of  the  present  study 
this  was  the  species  to  receive  the  most  attention  (William  Saunders, 
1875;  Fox,  1911).  These  earlier  examinations  demonstrated  the  pres- 
ence of  rubber,  but  in  small  amounts,  this  being  due  either  to  the 
chance  gathering  of  poor  strains  or  to  harvesting  at  the  wrong  season 
or  from  plants  grown  under  conditions  unfavorable  to  the  formation 
of  rubber.  The  percentage  content  was  not  determined  and  expressed 
in  a  sufficiently  definite  manner  to  render  comparison  with  the  tables 
possible.  The  analyses  here  reported  give  further  evidence  of  great 
variability  in  rubber-content  between  different  plants.  Whether  this 
is  due  to  hereditary  qualities  or  to  environmental  causes  is  still  a 
subject  for  experimentation.  As  a  beginning  on  this  problem  an 
analysis  was  made  of  green  leaves  for  comparison  with  fully  matured 
and  yellow  leaves  of  plants  growing  in  the  same  locality  and  under 
conditions  that  seemed  to  be  identical.  In  each  case  leaves  were 
taken  from  5  plants,  both  lower  and  upper  leaves  being  selected;  these 
were  dried,  pulverized,  and  thoroughly  mixed.  In  this  manner  it  was 


26 


KUBBER-CONTENT   OF  NORTH   AMERICAN   PLANTS. 


hoped  to  reduce  the  error  due  to  individual  variation,  although  it  is 
admitted  that  a  much  larger  number  would  give  more  convincing  data. 
The  results  as  shown  in  table  3  under  Nos.  1034  and  1035  indicate 
that  mature  leaves  contain  the  highest  percentage.  This  is  in  accord- 
ance with  the  analyses  reported  under  Nos.  826  and  827,  where  younger 
and  older  leaves  from  individual  plants  are  compared,  and  with  what 
has  been  suggested  from  other  species.  No.  1036  seems,  however,  to 
be  an  exception,  perhaps  accounted  for  by  the  pathologic  conditions 
of  the  foliage. 

Another  item  of  interest  brought  out  in  table  3  is  the  fact  that  larga, 
robust  plants,  like  No.  1032,  may  carry  as  high  or  higher  percentages 
of  rubber  in  their  leaves  as  average  or  small  ones.  The  practical 
application  of  this  is  self-evident.  In  common  with  the  other  species 
examined,  excepting  only  A.  subulata,  the  amount  of  rubber  in  the 
stems  is  almost  negligible. 

TABLE  B.^Chemical  analyses  of  Asclepias  syriaca. 


No. 

Place  of  collection. 

Date  of 
collection. 

Part  of  plant 
analyzed. 

Acetone 
extract. 

Rubber 
(benzene 
extract). 

826 

Madison  Nebr  

Aug.  27 

Leaves  *  

p.  ct. 

p.  ct. 
1  4 

Leaves  J     

2  4 

Sterna  J  

0  3 

Stems4  

0  21 

827 

do                 

do 

Leaves  *         

0  53 

Leaves*   

1  10 

Stems*       

0  27 

805 

Near  Humphrey,  Nebr.  '.  . 

Sept.  25 

Leaves       

8  8 

4  2 

Stems  

11.6 

0  2 

898 

Enola,  Nebr  

Sept.  24 

Stems  

6.5 

0.4 

Leaves  

9.8 

2  1 

1032 

Lincoln,  Nebr.  '  

Oct.   20 

Leaves  

10.6 

4  1 

Stems           

5.5 

0  3 

1034 

do         

..  do  

Leaves7   

10  2 

3  6 

Stems  

6.4 

0.3 

1035 

do  

...do.... 

Leaves  8  

11.8 

4.4 

Stems     

4.4 

0  4 

1036 

...  do  

...do  

Leaves  '   

8.6 

3  3 

Stems       

5.7 

0  5 

1  Young. 

1  Old  leaves  from  lower  down. 

1  Upper  portion. 

4  Lower  portion. 

*  Between  Humphrey  and  Madison. 


•  Very  large  plant;  leaves  just  turning  yellow. 
7  Dried  while  green;  mixture  from  5  plants. 
1  Dried  when  yellow;  mixture  from  5  plants. 
'  Dried  when  yellowing;  mixture  from  5  plants, 
partly  mildewed. 


ASCLEPIAS  CALIFORNICA. 
Synonym:  Gomphocarpus  tomentosus. 

Description. — Plant  a  rounded  perennial  herb,  2  to  3  feet  high  and 
about  as  broad;  roots  exceptionally  large,  branching  near  the  surface 
of  the  soil  to  form  a  broad  crown;  stems  usually  4  to  10,  unbranched, 


THE   MOST   IMPORTANT   SPECIES.  27 

straight,  not  erect,  but  spreading  or  ascending,  closely  covered  with 
matted  hairs  like  wool;  leaves  4  to  8  pairs  on  each  stem,  ovate  or 
oblong,  acuminate,  2.5  to  5  inches  long,  1.5  to  3  inches  wide,  covered 
like  the  stems  with  matted  white  hairs  which  are  somewhat  deciduous 
as  the  leaves  mature;  flowers  appearing  from  March  to  early  July, 
greenish  or  dull  purplish,  in  rounded  clusters  (umbels),  the  clusters 
pendent  from  the  axils  of  the  upper  leaves  and  ends  of  the  stems, 
differing  from  all  of  the  other  milkweeds  here  described  in  having 
neither  horn  nor  crest  to  the  hoods;  pods  on  bent  stalks,  pendent  or 
erect,  elongated  ovoid,  about  4  inches  long,  white  with  a  dense  wool- 
like  tomentum,  maturing  from  late  July  into  the  autumn. 

References. — Torrey,  Bot.  Mex.  Bound.,  160,  pi.  44,  1859  (as  Acerates  tomentosa).  Gray, 
Syn.  FL,  21: 100,  1878  (as  Gomphocarpus  tomentosus).  Hall,  Bot.  Gaz.,  31:  389,  1901. 

Distribution  and  ecology. — This  milkweed  grows  only  in  western 
California,  from  the  latitude  of  San  Francisco  south  to  the  Mexican 
border  and  on  the  westerly  slope  of  the  Sierra  Nevada  from  Mariposa 
County  to  Tehachepi  Pass.  It  grows  scatteringly  on  the  inner  Coast 
Ranges,  where  it  reaches  its  northern  limit  on  Mount  Diablo,  and  is 
common  only  in  southern  California.  The  largest  quantities  have 
been  found  on  the  foothill  slopes  along  Lytle  Creek  and  Cajon  Pass; 
on  sandy  washes  and  slopes  from  Corona  to  Elsinore  and  Temecula; 
and  again  on  the  foothills  of  San  Gorgonio  Pass  north  of  Banning. 
It  grows  also  on  the  desert  area,  as,  for  example,  near  Victorville, 
but  it  does  not  range  far  from  the  surrounding  mountains.  Although 
an  abundance  of  material  for  experimental  purposes  may  be  obtained 
from  these  localities,  the  species  is  nowhere  truly  abundant;  that  is, 
it  does  not  form  pure  stands  over  large  areas. 

The  ecologic  requirements  of  Asclepias  calif ornica  include  a  rather 
light,  somewhat  sandy  soil,  a  very  moderate  amount  of  moisture,  and 
this  only  during  the  early  spring,  and  warm,  clear  days  during  the 
growing-period.  The  total  annual  precipitation  at  some  of  the 
stations  mentioned  above  does  not  exceed  16  inches,  most  of  this  com- 
ing from  January  to  April.  The  temperature  falls  to  about  20°  F. 
in  winter  and  rises  to  about  110°  F.  in  summer.  The  plants  are 
usually  surrounded  by  a  sparse  growth  of  grasses  and  other  herbs; 
but  these  are  not  sufficiently  high  to  yield  shade.  Sometimes  the 
milkweeds  are  partially  shaded  by  shrubs,  but  the  species  grows  only 
along  the  edges  of  the  chaparral  formation  and  does  not  creep  up 
under  the  shrubs  for  protection. 

Judging  from  conditions  under  which  the  plants  grow  wild,  it  would 
seem  that  Asclepias  calif  ornica  could  be  brought  under  cultivation  on 
exceedingly  poor  land.  A  light,  well-drained  soil  should  be  selected 
and  a  location  where  winter  temperatures  do  not  fall  much  below  20°. 
High  summer  temperatures  are  quite  certainly  desirable,  since  the 


28 


RUBBER-CONTENT   OF  NORTH  AMERICAN  PLANTS. 


best  development  both  of  the  species  and  of  individuals  is  found  on  the 
hot  foothill  slopes  of  southern  California,  whereas  in  the  Coast  Ranges 
farther  north  the  plants  are  more  scattered  and  less  well  developed. 
Irrigation  would  not  be  essential,  although  in  some  districts  it  might 
be  practiced  during  the  winter  or  spring  in  order  to  stimulate  growth. 
On  the  other  hand,  an  excess  of  moisture  and  low  heavy  soil,  as  well  as 
alkali,  should  probably  be  avoided. 

A  feature  of  this  species  which  might  militate  against  its  use  is  its 
lack  of  an  erect  growing-habit.  Since  the  stems  spread  more  or  less 
from  the  base,  each  plant  requires  considerable  space  and  perhaps  would 
be  somewhat  difficult  to  catch  with  ordinary  mowing  machinery. 
However,  the  stems  never  lodge  or  lie  directly  on  the  ground,  and 
it  is  probable  that  they  would  approach  a  vertical  position  if  grown 
closely  together.  The  abundant  woolly  hairiness  of  the  stems  and 
leaves  should  also  be  taken  into  account,  since  this  might  interfere 
with  the  use  of  certain  types  of  machinery  used  in  preparing  the  plants 
for  extraction. 

TABLE  4. — Chemical  analyses  of  Asclepias  calif ornica. 


No. 

Place  of  collection. 

Date  of 
collection. 

Part  of  plant 
analyzed. 

Acetone 
extract. 

Rubber 
(benzene 
extract)  . 

350 

Banning,  Calif  

Aug.  25 

Leaves1  

p.  ct. 
6.9 

p.  ct, 
4.1 

Leaves  1  

8.0 

3.99 

Leaves*  

8.0 

3.77 

Stems  

9.7 

0.75 

Water-free  latex  

58.0 

10.0 

360 

Mount  Diablo,  Calif  

Oct.    13 

Leaves  * 

2.6 

919 

San  Benito  Co.,  Calif  

June     1 

Leaves  *   

7.0 

0.9 

Stems       

7.1 

0.7 

992 

Banning,  Calif  

May  .  . 

Leaves  *  .  .  .  .          

6.8 

1.2 

1  Good  condition.  2  Partly  molded. 

6  Young;  product  granular,  not  rubber-like. 


*  Old  and  drying. 


4  Very  old. 


Rubber-content. — The  results  of  the  chemical  examination  of  three 
collections  of  this  milkweed  exhibit  a  wide  range  of  variation.  This 
may  be  due  to  the  inclusion  of  several  strains  in  this  one  species,  but 
it  may  also  be  explained  on  other  grounds.  It  is  noted,  e.  g.,  that  the 
leaves  of  No.  919  were  very  old  and  dry,  that  those  of  No.  360  were  also 
past  maturity,  although  not  yet  hardened,  while  those  of  No.  350,  which 
gave  the  highest  yield,  were  about  at  the  stage  of  full  maturity. 

The  analysis  of  the  dried  latex  is  interesting,  since  it  is  one  of  the 
few  analyses  made  of  this  substance  as  it  occurs  in  Asclepias.  The 
results  are  considerably  higher  than  those  given  by  Fox  (1911)  for 
Asclepias  syriaca,  and  also  higher  than  those  found  in  tropical  latex 
plants,  such  as  Hevea  brasiliensis. 


THE    MOST   IMPORTANT   SPECIES.  29 

ASCLEPIAS  LATIFOLIA    (BROAD-LEAF  MlLKWEED). 
Synonym:  Asclepias  jamesi. 

Description. — Plant  a  stout,  very  leafy  perennial  herb,  usually  1 
to  2  but  sometimes  3  feet  high;  roots  coarse  and  woody;  stems  several, 
some  times  much  crowded  and  forming  bush-like  plants,  unbranched, 
either  erect  or  spreading,  not  hairy;  leaves  ample,  commonly  12  to  20 
but  often  30  on  each  stem,  oval  to  orbicular,  somewhat  heart-shaped  at 
base,  4  to  6  inches  long  and  nearly  as  wide,  very  thick,  minutely  hairy  when 
young  but  soon  becoming  smooth  and  green;  flowers  greenish,  appear- 
ing from  June  to  August,  borne  in  dense,  short-stalked  clusters  from 
the  axils  of  the  upper  leaves;  pods  erect  on  curved  stalks,  ovoid,  2  to  3 
inches  long,  about  1  inch  thick,  maturing  in  September  and  October. 

References.— Gray,  Syn.  Fl.,  2»:92,  1878  (as  A.  jamesi).     Britton  and  Brown,  111.  Fl.' 
ed.  2,  27:  fig.  3391,  1913. 

Distribution  and  ecology. — The  broad-leaf  milkweed  is  an  inhabitant 
of  the  plains  and  lower  foothills  from  Nebraska  and  Colorado  south  and 
west  to  Kansas,  Texas,  and  northwestern  Arizona.  It  belongs  to  the 
pinon-juniper  association  and  always  grows,  so  far  as  we  have  been 
able  to  observe,  in  small,  widely  separated  clumps,  these  occupying 
several  square  feet  and  with  stems  probably  all  from  one  root.  Along 
the  eastern  base  of  the  Rocky  Mountains  it  usually  occupies  the 
summits  of  low,  rounded  hills,  or  grows  scatteringly  over  warm,  dry 
southerly  slopes  where  the  soil  is  well  drained.  Specific  localities 
include  hills  south  of  Hays,  Kansas;  Ashfork  and  near  Williams, 
Arizona;  and  the  following,  all  in  Colorado:  Canyon  City,  low  hills 
north  of  Walsenberg,  and  south  slopes  above  the  Purgatoire  River 
just  west  of  Trinidad.  Records  of  rainfall  and  temperature  at  all  of 
these  stations  are  not  at  hand,  but  at  Trinidad,  which  may  be  taken 
as  representing  the  slopes  along  the  easterly  base  of  the  Colorado 
Rockies,  where  the  species  is  abundant,  the  normal  annual  precipita- 
tion is  17  inches,  the  lowest  recorded  temperature  is  —26°  F.,  and  the 
highest  recorded  temperature  101°  F.  At  Pueblo,  in  the  same  general 
district,  the  temperatures  are  about  the  same,  but  the  normal  precipi- 
tation is  slightly  under  12  inches. 

In  this  species  we  have  a  form  which  is  not  exacting  in  its  require- 
ments. To  all  appearances  it  has  been  driven  to  the  habitats  just 
described  because  of  competition  with  other  plants  where  the  soil 
and  moisture  conditions  are  more  favorable.  If  this  has  been  the 
case,  then  if  brought  under  cultivation  on  good  soils  and  competition 
removed  through  proper  tillage  the  plants  would  doubtless  respond 
with  a  greatly  increased  vigor  and  growth.  While  this  would  increase 
the  tonnage,  the  percentage  of  rubber  in  the  plant  might  also  be  affected 
and  perhaps  disastrously.  Only  by  experiment  can  this  point  be 
positively  determined.  Aside  from  experimenting  with  this  species 


30 


RUBBER-CONTENT   OF  NORTH   AMERICAN  PLANTS. 


on  good  soils,  it  should  be  tried  also  on  sandy  or  stony  slopes,  since 
such  lands  would  be  less  expensive  and  the  natural  habitat  indicates 
that  growth  can  be  made  under  such  conditions.  As  in  the  case  of 
Asclepias  calif ornica,  the  tendency  of  the  plants  to  spread  out  close 
to  the  ground  may  interfere  somewhat  with  mowing,  although  close 
planting  might  correct  this  tendency.  Field  observations  indicate 
that  there  is  considerable  variation  in  the  habit,  the  stems  in  some 
localities  tending  to  spread,  in  others  to  assume  a  strictly  upright 
position.  The  best-formed  plants  seen  during  the  present  study  were 
found  south  of  Pueblo,  Colorado.  Here  each  individual  formed 
a  dense  leafy  clump  3  feet  high  and  about  4  feet  across.  The  excep- 
tionally large  size  of  the  roots  of  this  species  augurs  well  for  its  ability 
to  send  up  new  shoots  after  a  crop  has  been  gathered. 

Rubber-content. — In  considering  table  5,  Nos.  342  and  914  may  be 
ignored,  since  the  former  includes  very  old  leaves  and  the  latter  only 
stems.  This  leaves  2  to  3.8  per  cent  for  the  rubber-content  as  far  as 
known.  A  study  of  the  weight  of  the  foliage  and  of  probable  yield  of 
rubber  per  acre  has  been  made  on  the  basis  of  material  gathered  at 
Ashfork,  Arizona,  under  Nos.  928  and  953,  as  follows :  average  number 
of  leaves  per  stem,  24;  average  dry  weight  of  single  leaves,  0.037 
ounce;  average  number  of  stems  per  square  foot,  4;  computed  weight 
of  dry  leaves  per  acre,  9,392  pounds.  On  the  basis  of  a  3  per  cent 
analysis  this  would  yield  281  pounds  of  rubber  per  acre.  It  is  evident 
that  a  yield  so  low  as  this  would  not  justify  one  in  growing  this  plant 
for  rubber  even  on  cheap  land,  and  that  other  species,  notably  Asclepias 
subulata,  are  much  more  promising  as  rubber  producers,  based  upon 
our  present  knowledge.  The  figures  indicate,  however,  only  the  yield 
from  the  unimproved  form.  Attempts  to  increase  the  yield  should 
aim  at  the  development  of  a  more  erect  and  taller  grower,  as  well  as 
of  a  strain  with  increased  rubber-content. 

TABLE  5. — Chemical  analyses  of  Asclepias  latifolia. 


No. 

Place  of  collection. 

Date  of 
collection. 

Part  of  plant 
analyzed. 

Acetone 
extract. 

Rubber 
(benzene 
extract). 

824 

Hays,  Kans  

Sept.  .  . 

Stems  

p.  ct. 

p.  ct. 
0.64 

928 

Ashfork,  Ariz  

Aug.  22 

Leaves  

7.1 

2.5 

Stems  

8.3 

0.63 

342 

Near  T^alsenberg,  Colo.  .  . 

Aug.  16 

Leaves  *   

8.0 

3.7 

Leaves*     

9.0 

1.0 

800 

Near  Dodge  City,  Kans 

Sept.  .  . 

Leaves       

7.0 

3.8 

Stems         

7.0 

0.7 

880 

Royal  Gorge  Colo.    .  .    . 

Aug.  30 

Leaves       

8  0 

2.0 

Stems  

10.1 

0.7 

913 

Magdalena,  N.  Mex  

Oct.    13 

Leaves   

7.4 

3.3 

914 

do  

...do  

Stems     

11.0 

0.5 

953 

Ashfork,  Ariz  

Sept.  28 

Leaves   

8.1 

2.1 

Stems  

8.5 

0.7 

1  Medium  material.     Leaves  of  this  number  from  several  plants. 


1  Very  old. 


THE   MOST   IMPORTANT   SPECIES.  31 

ASCLEPIAS   MEXICANA. 

Description. — A  perennial  erect  herb,  woody  at  the  base,  2  to  4 
feet  or  in  moist  situations  to  over  6  feet  high,  usually  narrow  and 
slender-stemmed,  but  up  to  8  feet  in  diameter  in  vigorous  forms, 
especially  in  alkaline  soil,  sometimes  spreading  by  the  growth  of  the 
roots  to  form  beds  15  feet  or  more  across;  roots  connected  by  deep- 
seated  underground  horizontal  branches ;  stems  usually  few  at  a  place, 
but  under  favorable  conditions  as  many  as  80  to  100  are  crowded  into 
a  single  clump,  straight,  erect,  mostly  unbranched,  green  and  glabrous; 
leaves  40  to  80  on  each  stem,  arranged  in  whorls  of  3  or  6  each,  linear, 
tapering  to  the  apex,  2.5  to  6  inches  long,  0.25  to  0.5  inch  wide,  green 
and  glabrous  like  the  stems;  flowers  appearing  in  summer,  dull  white, 
smaller  than  in  most  other  species,  in  small  rounded  clusters  on  erect 
peduncles  from  axils  of  upper  leaves;  pods  on  curved  or  straight  stalks, 
narrowly  ovoid,  tapering  to  apex,  2.5  to  3.5  inches  long,  about  0.5  inch 
thick,  minutely  pubescent,  the  seeds  ripening  from  late  July  to  October. 

References.— Cavanilles,  Icon.,  1:42,  pi.  58,  1791.  Gray,  Syn.  FL,  21:96,  1878.  Hall 
and  Yates,  Calif.  Agr.  Exp.  Sta.  Bull.  249,  fig.  5,  1915. 

Distribution  and  ecology. — The  range  of  this  species  extends  from 
Mexico,  whence  came  the  original  specimens,  to  Arizona,  Nevada, 
California,  northern  Idaho,  and  southern  Washington.  It  grows  in 
greatest  abundance  and  to  the  largest  size  in  the  hot  interior  valleys 
of  California,  but  plenty  of  vigorous  plants  are  also  to  be  found  in  the 
more  elevated  valleys  of  western  Nevada,  such  as  those  of  the  Carson 
and  Truckee  Rivers.  The  largest  plants  are  always  found  in  moder- 
ately alkaline  soil  and  are  often  associated  with  such  halophytes  as 
Sporobolus  airoides  and  Distichlis  spicata.  These  facts  suggest  that 
the  proper  place  for  the  cultivation  of  the  plant  on  a  large  scale  would 
be  the  vast  expanses  of  territory  in  the  San  Joaquin  Valley  of  California 
and  the  valleys  of  western  Nevada  which  are  now  uncultivated  either 
because  of  alkaline  conditions  or  the  lack  of  water  for  irrigation. 

An  ecologic  feature  especially  noticeable  in  this  species  is  the  prompt- 
ness with  which  new  stems  are  sent  up  after  the  old  ones  have  been 
removed.  It  is  certain  that  under  ordinary  conditions  two  crops 
could  be  harvested  in  a  year,  and  this  probably  without  any  reduction 
in  the  percentage  content  of  rubber  (see  p.  52).  Even  a  third  crop 
seems  not  impossible  if  the  roots  receive  a  reasonable  amount  of 
moisture.  In  the  single  instance  where  this  was  attempted  the 
locality  and  the  season  were  so  unfavorable  that  only  a  sparse  growth 
resulted.  The  experiment  should  be  repeated  in  one  of  the  moist, 
alkaline  valleys  where  the  plants  exhibit  then*  maximum  vigor.  It 
must  be  remembered,  however,  that  excessive  cropping  might  so 
weaken  the  roots  as  to  diminish  the  growth  of  the  year  following. 

Rubber-content. — The  chemical  analyses  of  Asclepias  mexicana 
indicate  that  there  is  a  wide  range  of  variation  in  rubber-content 


32 


RUBBER-CONTENT  OF  NORTH  AMERICAN   PLANTS. 


between  the  different  samples  tested,  with  the  highest  one  yielding  4.8 
per  cent  from  the  leaves.  It  seems  impossible  to  correlate  this  varia- 
tion either  with  geographic  distribution  or  ecologic  conditions.  The 
inference,  therefore,  is  that  the  species  comprises  a  considerable  num- 
ber of  genetic  strains,  that  further  exploration  may  discover  better 
ones  than  any  thus  far  found,  and  that  breeding  experiments  with  so 
large  a  number  to  select  from  might  yield  surprising  results.  On  the 
other  hand,  the  foliage  is  more  sparse  than  in  most  other  milkweeds 
and  constitutes  only  a  minor  portion  of  the  total  weight  of  the  plant. 
A  form  in  which  the  stems  as  well  as  the  leaves  would  carry  a  high 
rubber-content  is  therefore  desirable.  The  possibility  of  producing 
such  a  strain  is  indicated  by  sample  No.  918,  from  alkaline  soil  of  the 
San  Joaquin  Valley,  in  which  the  stems  yielded  2.3  per  cent  at  the 
same  time  that  the  leaves  carried  4.4  per  cent.  If  it  is  found  that  these 
figures  can  be  increased  by  scientific  breeding  and  the  plants  grown  on 
alkaline  soil  to  the  size  and  density  of  that  shown  on  plate  3,  Asclepias 
mexicana  will  take  its  place  as  one  of  the  most  promising  species. 

TABLE  6. — Chemical  analyses  of  Asclepias  mexicana. 


No. 

Place  of  collection. 

Date'of 
collection. 

Part  of  plant 
analyzed. 

Acetone 
extract. 

Rubber 
(benzene 
extract)  . 

356 

Sept  23 

Leaves  

p.  ct. 

p.  ct. 
2  6 

Stems  

0  75 

358 

Oct    12 

Leaves  

2  7 

359 

Walnut  Creek  Cajif 

Oct    13 

Leaves  

4  8 

Stems  

0  68 

905 

The  Dalles  Ore 

Sept  25 

Leaves  

8.9 

1  4 

Stems  

6.6 

0  5 

909 

Woodland  Calif 

Oct      5 

Leaves  

10.0 

1  8 

Stems  

8.7 

0  6 

910 

Mount  Diablo  Calif  1- 

Oct    11 

Leaves           

10  3 

3  2 

Stems  

6  6 

0  6 

911 

do  1 

do 

Leaves       

11  0 

4  1 

Stems  

7.1 

0  84 

912 

do.  i                   

do 

Leaves  

12.3 

2  8 

Stems  

8  3 

0  63 

916 

Tehachepi    Calif  * 

Oct    22 

Leaves     

12  5 

4  0 

Stems     

6  4 

0  5 

918 

Bakersfield   Calif  2 

Oct    23 

11  0 

4  4 

Stems  

12  9 

2  3 

923 

Solano  Co.,  Calif.  3 

Nov.  13 

Leaves  

8  9 

4  1 

924 

Nov   14 

Leaves   

11  1 

3  5 

941 

do.  B            

July     9 

Leaves  

8  2 

3.7 

Stems     

10  9 

1  0 

Whole  6  

2  1 

942 

...  do           

do 

Leaves  7  

14  0 

2  2 

Stems       

11  5 

0  7 

1037 

do.8  

Oct.   12 

Leaves  

11  1 

4  6 

Stems     

5  5 

0  8 

Whole  6     

2  4 

1038 

do.9  

...do  

Whole  

9.0 

1.5 

1  Single  plant. 

2  Soils  trongly  alkaline. 

3  Second  growth;  mixture  of  leaves  from  80  plants. 

4  Mixture  of  leaves  of  40  plants. 
6  Second  growth. 


8  Computed. 

7  Second  growth;  mixture  from  40  plants. 

8  Second  growth  from  areas  adjacent  to 

941  and  1038. 

9  Third  growth  from  same  roots  as  941. 


THE    MOST   IMPORTANT   SPECIES.  33 

ASCLEPIAS    GALIOIDES    (WHORLED    MlLKWEED). 

Description. — An  erect  perennial  herb,  sometimes  slightly  woody  at 
the  base,  1  to  3  feet  high;  roots  erect  and  much  branched  just  below 
the  surface,  but  these  crowns  connected  below  by  horizontal  root- 
branches;  stems  few  to  numerous  at  a  place,  straight,  unbranched, 
green,  and  glabrous;  leaves  20  to  60  on  each  stem,  in  whorls  of  3 
or  6  each  or  some  of  the  lower  only  in  pairs,  linear,  tapering  to  the 
apex,  2  to  4  inches  long,  about  0.8  inch  wide,  green  and  smooth  on 
both  sides;  flowers  appearing  from  June  to  August,  dull  white,  small, 
in  small  stalked  umbels  from  the  axils  of  the  upper  leaves ;  pods  strictly 
erect,  on  straight  stalks,  fusiform,  much  narrowed  above,  3  to  4  inches 
long,  scarcely  0.5  inch  thick,  glabrous  and  smooth,  the  seeds  ripening 
in  August  and  September. 

References.— Britton  and  Brown,  111.,  Fl.,  ed.  2:  32,  fig.  3405, 1913.     Marsh  et  al.,  U.  S 
Dept.  Agr.  Bull.  800,  pis.  1  to  3, 1920. 

Distribution  and  ecology. — This  is  a  species  of  the  southern  Rocky 
Mountain  region,  from  middle  Utah  and  middle  Colorado  (Glenwood 
Springs  and  valley  of  the  Arkansas),  south  into  Mexico  and  Central 
America.  It  seems  to  be  most  abundant  in  Arizona  and  New  Mexico. 
Specific  localities  where  it  may  be  obtained  in  abundance  include  El 
Paso  in  Texas  and  Flagstaff  and  Holbrook  in  Arizona.  The  natural 
habitat  of  this  plant  is  on  the  dry  plains  and  foothills,  often  in  sandy 
soil.  It  is  abundant  on  overgrazed  areas,  where  it  often  replaces  the 
original  grasses  and  leguminous  plants,  and  also  in  fallow  fields.  It 
responds  quickly  to  the  influence  of  cultivation  or  irrigation,  as  along 
ditch-banks,  and  then  makes  a  rank  growth,  spreading  rapidly  over 
large  areas.  The  downy  seeds  are  carried  both  by  wind  and  by  irriga- 
tion-water, while  in  cultivated  fields  the  horizontal  roots  are  broken 
up  and  distributed,  each  piece  giving  rise  to  a  new  plant.  When  this 
milkweed  gets  a  start  in  neglected  orchards  it  sometimes  forms  a  solid 
growth  between  the  trees.  Its  poisonous  nature  has  been  described 
by  Marsh  et  al.  (1920). 

Asclepias  galioides  has  been  much  confused  with  a  more  easterly 
species,  namely,  A.  verticillata,  a  species  which  is  sharply  set  off  by 
the  whorls  of  numerous  fibrous  adventitious  roots  from  the  lower 
nodes  of  the  stem  and  by  other  characters,  as  recently  emphasized 
by  Eggleston  (in  Marsh  et  al.,  1920).  It  is,  in  reality,  much  more 
closely  related  to  A.  mexicana,  from  which  it  is  scarcely  distinguish- 
able, except  by  its  usually  narrower  leaves.  Also  in  the  matter  of  its 
rubber-content  and  in  its  ecologic  requirements,  A.  galioides  closely 
resembles  A.  mexicana,  and  it  is  not  unlikely  that  it  should  be  treated 
as  a  geographic  variety  of  this  more  westerly  species. 

Rubber-content. — The  small  number  of  analyses  made  of  this 
species  does  not  warrant  the  drawing  of  definite  conclusions  as  to  the 


34 


RUBBER-CONTENT   OF   NORTH  AMERICAN   PLANTS. 


amount  of  rubber  present.  The  somewhat  high  yield  of  5.2  per  cent 
in  the  leaves  of  one  collection  seems  to  justify  the  hope,  however,  that 
further  examinations  would  show  that  it  would  average  at  least  as 
high  as  in  A.  mexicana.  These  two  species  are  so  closely  similar  in 
their  botanical  characters  that  the  extent  of  variation  found  in  mexi- 
cana may  be  reasonably  predicted  for  galioides.  A  cross  between  the 
two,  using  the  individuals  with  the  highest  rubber-content  in  each  case, 
might  bring  results  of  interest. 

TABLE  7. — Chemical  analyses  of  Asclepias  galioides. 


No. 

Place  of  collection. 

Date  of 
collection. 

Part  of  plant 
analyzed. 

Acetone 
extract. 

Rubber 
(benzene 
extract). 

338 

Purgatoire  River,  N.  M  .  . 

Aug.  15 

Leaves  

p.  ct. 
10.7 

•p.  ct. 
0.62 

842 

Las  Cruces,  N.  M  ;  .  . 

Sept.  .  . 

Leaves  

5.2 

Stems  

0.85 

879 

Canyon  City,  Colo  

Aug.  30 

Whole     

6  0 

1.5 

ASCLEPIAS    BRACHYSTEPHANA. 

Description. — Plant  a  comparatively  slender  leafy  herb,  1  to  2  feet 
high;  roots  thick  and  woody,  producing  clusters  of  stems,  the  clusters 
perhaps  connected  by  deep  horizontal  cross-roots;  stems  usually 
branched  near  the  base,  but  otherwise  mostly  simple  and  straight, 
erect,  very  smooth,  and  without  hairs;  leaves  numerous  to  the  top, 
commonly  15  to  30  on  each  shoot,  all  opposite,  narrowly  lanceolate, 
2  to  5  inches  long,  0.25  to  0.5  inch  wide,  rather  thin,  smooth,  veiny, 
not  hairy;  flowers  pink  or  dull  purple,  small,  appearing  from  April 
to  August,  or  even  September  after  summer  rains,  in  small,  short- 
stalked  clusters  among  the  upper  leaves;  pods  on  sharply  deflected 
stalks,  erect,  narrowly  ovoid,  long-pointed,  1.5  to  3  inches  long,  about 
0.75  inch  thick  when  mature,  covered  when  young  with  a  fine  gray 
scurfy  tomentum,  but  smooth  and  nearly  glabrous  in  age,  maturing 
from  middle  July  to  October. 

References.— Gray,  Syn.  Fl.,  2^94,  1878.  Britton  and  Brown,  111.  Fl.,  ed.  2:31,  fig. 
3402,  1913. 

Distribution  and  ecology. — This  species  is  most  abundant  in  New 
Mexico,  but  ranges  west  almost  throughout  Arizona,  where  it  is  much 
less  common,  and  southeast  into  western  Texas.  It  occurs  also  in 
northern  Mexico.  Specific  localities  include  the  forestry  experiment 
station  and  the  Turney  pasture,  both  near  Las  Cruces,  New  Mexico. 
The  plants  have  rather  well  developed  xerophytic  characters  and  grow 
on  dry  mesas  and  foothill  slopes.  They  are  especially  common  on 
overgrazed  or  otherwise  disturbed  areas. 


THE   MOST   IMPORTANT   SPECIES. 


35 


The  small  size  of  the  plant  is  somewhat  compensated  for  by  the 
abundant,  crowded  foliage,  but  the  leaves  are  so  narrow  that  a  large 
yield  of  herbage  can  scarcely  be  expected.  Seemingly  the  only  possi- 
bility of  this  species  taking  important  rank  will  depend  upon  the 
development  of  larger  plants  under  cultivation  and  the  maintenance 
of  the  unique  quality  of  carrying  as  much  rubber  in  the  stems  as  in  the 
leaves,  as  will  be  indicated  below. 

Rubber-content. — It  will  be  noted  from  table  8  that  all  of  the  plants 
analyzed  came  from  one  general  locality.  Little  can  be  said,  there- 
fore, regarding  the  rubber-content  of  the  species  as  a  whole.  The 
most  remarkable  feature  of  the  results  here  tabulated  is  the  almost 
equivalent  percentages  of  rubber  in  stems  and  leaves  in  3  out  of  the  4 
samples.  Even  in  the  fourth  the  difference  is  not  so  great  as  is  found 
in  other  species.  It  must  be  added,  however,  that  the  analyses  given 
under  "whole"  are  not  of  the  same  individual  plants  as  those  reported 
under  "leaves"  for  the  same  number.  In  the  first  two  numbers  the 
leaves  and  stems  are  from  the  same  plants  in  each  case.  The  word 
"whole"  as  used  in  the  table  is  somewhat  misleading,  since  the  roots 
were  not  included  in  any  case.  It  should  therefore  be  taken  to  mean 
stems  and  leaves  as  ground  up  and  analyzed  together.  The  results  of 
Nos.  884  and  885  indicate  that  mature  plants  contain  a  higher  per- 
centage of  rubber  than  immature  ones.  This  is  in  accordance  with 
the  findings  for  other  species. 

TABLE  8. — Chemical  analyses  of  Asclepias  brachystephana. 


No. 

Place  of  collection. 

Date  of 
collection. 

Part  of  plant 
analyzed. 

Acetone 
extract. 

Rubber 
(benzene 
extract). 

801 

Las  Cruces,  N,  Mex.  *  ... 

Sept.  .  . 

Leaves  

p.  ct. 
9.0 

p.  ct. 

2.7 

Stems  2       

7  0 

2.9 

823 

do.  3 

do  ... 

Leaves     

2.4 

Stems  2  

1.7 

884 

do.4  

Aug.  31 

Leaves  

8.0 

2.1 

Whole5     

7.8 

2.1 

885 

do.  *              

do     .. 

Leaves       

8.0 

2.9 

Whole  5  

8.0 

3.0 

1  Growing  in  pastures.       2  Same  plant.        3  Growing  in  yards.        4  Plants  in  blossom. 
5  Other  plants.  6  Plants  in  fruit. 

ASCLEPIAS  SPECIOSA  (SHOWY  MILKWEED). 

Description. — Plant  a  robust,  erect  perennial  herb,  4  to  6  feet  high; 
roots  connected  by  deep  horizontal  branches,  the  vertical  roots 
branched  near  the  surface  to  form  a  crown;  stems  several  to  numerous 
from  the  crown,  sometimes  forming  clumps  5  feet  or  more  in  diameter, 


36  RUBBER-CONTENT  OF  NORTH  AMERICAN   PLANTS. 

simple,  straight,  covered  with  a  fine  down  which  is  deciduous,  leaving 
the  old  bark  smooth;  leaves  numerous  to  the  top,  10  to  20  or  perhaps 
more  to  each  stem,  either  all  in  pairs  or  some  of  the  upper  ones  in 
whorls  of  3,  broadly  lanceolate  or  oblong-ovate,  4  to  6  inches  long,  2  to  3 
inches  wide,  thick,  finely  gray-tomentose  on  both  sides,  or  greener  and 
nearly  smooth  above;  flowers  flesh-color,  large,  and  showy,  appearing 
from  June  to  August,  in  stout-stalked  umbels  from  the  upper  leaf- 
axils  and  terminal;  pods  on  deflexed  stalks,  either  erect  or  pendent, 
with  an  ovoid  base  but  prolonged  into  a  stout  beak,  3.5  to  4  inches 
long,  1  to  1.25  inches  thick,  covered  with  a  gray  felt-like  tomentum, 
through  which  project  numerous  short,  soft  processes,  maturing  in 
August  and  September. 

References—  Curtis,  Bot.  Mag.,  pi.  4413, 1848.     Gray,  Syn.  Fl.,  21: 91, 1878.     Britton 
and  Brown,  111.  Fl.,  ed.  2:  30,  fig.  3399,  1913. 

Distribution  and  ecology. — This  is  the  most  widely  distributed  of  all 
of  the  milkweeds  of  western  North  America.  It  ranges  from  Alberta 
and  Minnesota  to  Iowa,  Texas,  Arizona,  California,  and  British 
Columbia.  The  vertical  distribution  is  also  remarkable,  since  it  grows 
from  the  hot,  low,  interior  valleys,  as  at  St.  George,  Utah,  and  Caliente, 
southern  Nevada,  to  the  moderately  cool  mountain  meadows  in  the 
lower  part  of  the  pine  belt  (Transition  Zone),  where  it  is  more  abundant, 
reaching  altitudes  of  over  8,000  feet  in  New  Mexico.  It  is  always 
partial  to  moist  soil  and  is  therefore  found  in  seepage  areas  along 
ditches  and  creeks  and  in  moist  meadows. 

It  is  readily  seen  from  the  facts  of  distribution  and  habitat  that  the 
showy  milkweed  is  a  species  which  could  be  grown  under  a  wider 
range  of  temperature  conditions  than  any  other.  It  is  also  to  be 
noted,  on  the  other  hand,  that  it  is  very  exacting  in  its  moisture 
requirements  and  that  probably  it  could  not  be  grown  to  perfection, 
except  in  moist  and  fairly  good  soil.  Sandy  soil  would  doubtless  be 
satisfactory  if  kept  moist  and  free  from  excessive  alkali.  These  condi- 
tions, together  with  the  usually  small  percentage  of  rubber  carried 
by  the  plant,  do  not  indicate  this  as  a  promising  species  for  further 
trial. 

Rubber-content. — The  earlier  analyses  by  Widtsoe  and  Hirst  have 
been  already  referred  to  (p.  10).  The  discouragingly  low  yields 
obtained  by  these  workers  are  practically  duplicated  by  most  of  the 
analyses  reported  upon  in  table  9.  The  amount  of  rubber  in  the 
leaves  reached  as  high  as  3  per  cent  in  only  2  samples  out  of  the  7 
examined.  It  is  possible  that  the  analysis  of  a  large  series  from  widely 
separated  localities  will  disclose  some  high-percentage  plants,  but 
Asclepias  spedosa  does  not  hold  out  as  much  promise  in  this  respect  as 
some  other  species  of  similar  habit. 


THE   MOST   IMPORTANT   SPECIES. 
TABLE  9. — Chemical  analyses  of  Asclepias  speciosa. 


37 


No. 

Place  of  collection. 

Date  of 
collection. 

Part  of  plant 
analyzed. 

Acetone 
extract. 

Rubber 
(benzene 
extract). 

304 

Manitou   Colo.       .  . 

July  18 

Leaves  

p.  ct. 
13  9 

p.  ct. 
0  99 

Stems  

8.4 

0.14 

Roots  

9.9 

0.41 

365 

Vaca  Valley,  Calif  

Oct.   27 

Leaves  

3.0 

370 

Near  Manitou,  Colo.  *  ... 

July   14 

Leiaves   

7.8 

1.6 

Stems       

6  6 

2  6 

371 

Colorado  Springs,  Colo.  *  . 

.  .  do 

Leaves   

5  9 

1  4 

Stems  

7.1 

0.19 

388 

Fort  Scott,  Kans  

Aug.    9 

Leaves  

7.5  ' 

2.1 

399 

Ute  Pass,  Colo.  

Aug.  20 

Leaves  

8.7 

2.6 

Stems     

7.1 

0.7 

889 

Wichita,  Kans  

Sept.    2 

Leaves   

7.0 

3  0 

Stems             

6  0 

0  47 

991 

Colorado  Springs,  Colo  .  .  . 

Latex  

67.5 

2.1 

1  A  broad-leaf  form. 


*  A  narrow-leaf  form. 


APOCYNUM  CANNABINUM  (INDIAN  HEMP)  AND  A.  ANDROS^JMIFOLIUM 
(SPREADING  DOGBANE). 

Description  (of  A.  canndbinum). — Plant  an  erect  perennial  herb, 
3  to  6  feet  high;  roots  connected  by  horizontal  creeping  rootstocks,  a 
single  plant  thus  coming  to  occupy  areas  several  feet  across  under 
favorable  conditions;  stems  several  at  a  place  or  solitary,  straight, 
smooth,  and  usually  without  hairs,  the  bast  yielding  a  fiber  of  fair 
quality  (p.  57) ;  leaves  numerous,  except  on  the  lower  part  of  old  stems, 
opposite,  short-petioled,  elliptic,  acute,  1.5  to  4  inches  long,  0.5  to  1.5 
inches  wide,  smooth,  glabrous  or  puberulent  in  some  varieties,  veiny; 
flowers  pinkish,  appearing  in  June,  July,  and  August,  small,  in  terminal 
and  lateral  stalked  cymose  clusters,  yielding  a  moderate  amount  of 
nectar  to  honeybees;  pods  pendent,  2  on  each  recurved  stalk,  terete, 
5  to  7  inches  long,  about  0.12  inch  thick,  smooth  and  without  hairs, 
maturing  from  July  to  September,  the  earlier  ones  often  well  formed 
before  the  last  flowers  have  fallen.  (Family  Apocynacese.) 

References.— Gray,  Syn.  Fl.,  21:  83, 1878.  Dodge,  U.  S.  Dept.  Agr.  Fiber  Investig.  Kept. 
6:  46,  pi.  5, 1894.  Dodge,  ibid.,  9:  62,  fig.  20,  1897.  Britton  and  Brown,  111.  FL,  ed.  2:  22, 
figs.  3378-3381,  1913. 

Distribution  and  ecology. — The  Indian  hemp  is  a  common  plant 
throughout  the  United  States  and  southern  Canada.  In  altitudinal 
range  it  extends  from  the  warm  plains  of  Florida  and  the  valleys  of 
southern  California  to  at  least  6,000  feet  in  the  western  mountains,  but 
it  is  not  known  with  certainty  above  the  lower  part  of  the  yellow-pine 
belt.  It  grows  best  in  gravelly  or  sandy  soil,  sometimes  where  but  few 
other  plants  can  grow,  but  it  requires  a  fair  amount  of  soil-moisture. 

The  cultivation  of  Indian  hemp  would  not  be  a  difficult  matter,  since 
it  propagates  abundantly,  both  by  seeds,  which  are  provided  with  tufts 


38  RUBBER-CONTENT   OF  NORTH   AMERICAN   PLANTS. 

of  long,  silky  down  and  are  therefore  carried  considerable  distances  by 
wind  and  water,  and  also  by  creeping  rootstocks.  When  these  root- 
stocks  are  divided,  as  in  plowing,  each  piece  gives  rise  to  a  new  plant. 
The  proper  place  for  experimental  plantings  would  be  in  moist  sandy 
soil  unsuited  to  agricultural  crops.  The  other  species  of  Apocynum 
here  considered,  that  is,  A.  androsoemifolium,  is  a  usually  smaller  plant 
with  more  spreading  branches  and  grows  in  drier  soils.  Nothing  has 
been  thus  far  discovered  that  brings  it  into  the  list  of  promising  plants 
for  rubber  culture. 

Rubber-content. — The  Indian  hemp  is  considered  as  one  of  the 
most  promising  plants  for  further  investigation.  It  is  true  that  most 
of  the  analyses  indicate  only  small  percentages  of  rubber,  but  the  high 
yield  of  No.  1039,  together  with  the  ease  of  vegetative  propagation  in 
poor,  sandy  soils  and  the  high  quality  of  the  product  greatly  modifies 
our  opinion  of  its  possibilities.  It  is  not  unlikely  that  further  search 
will  discover  still  better  plants,  and  any  strain  which  may  be  developed 
through  breeding  can  be  perpetuated  through  the  use  of  rootstocks. 
The  foliage  is  too  sparse  and  thin  to  yield  much  tonnage,  but  this  is 
somewhat  compensated  by  the  large  size  to  which  the  plants  grow 
and  the  density  of  the  stand  un.der  favorable  conditions. 

The  first  studies  of  the  latex  of  Apocynum  were  by  Fox  (1912).  He 
worked  especially  with  A.  androscemifolium,  but  this  and  the  larger 
A.  cannabinum,  or  true  Indian  hemp,  are  so  much  alike  in  other 
respects  that  the  latex  and  rubber  of  the  two  are  doubtless  quite 
similar.  According  to  this  investigator,  the  latex  is  coagulated  neither 
by  acids  nor  by  alkalies,  but  it  coagulates  slowly  and  slightly  on  boiling, 
and  immediately  and  completely  by  the  addition  of  acetone  in  the 
proportion  of  1  : 10  by  volume.  He  reports  further  that  formaldehyde 
coagulates  it  readily  but  much  more  slowly  than  acetone,  and  that  the 
latex  is  coagulated  by  phenol,  this  giving  a  soft  product.  Fox  also 
tried  a  salt  solution  and  found  that  this  coagulated  the  latex  slowly, 
giving  a  finely  divided  precipitate  hard  to  coalesce.  Boiling  the  salt 
solution  gave  a  soft  product.  Of  all  the  methods  tried,  the  use  of 
acetone,  perhaps  with  formaldehyde,  was  the  one  especially  recom- 
mended as  giving  the  best  results.  The  condition  of  the  soil  in  which 
the  plants  are  grown  exerts  an  influence  upon  the  amount  and  quality 
of  the  rubber  in  the  latex,  according  to  Fox.  He  found  that  plants 
grown  upon  dry,  sandy  soil  at  West  Akron,  Ohio,  gave  a  latex  contain- 
ing 2.27  per  cent  of  rubber  and  20.69  per  cent  of  resin,  whereas  plants 
grown  in  the  swamps  of  South  Akron  contained  1.12  per  cent  of  rubber 
and  15.04  per  cent  of  resin;  also  that  rubber  from  dry-grown  plants  is 
of  better  quality  than  that  of  wet-grown  plants.  Because  of  more 
recent  discoveries  as  to  the  great  extent  of  individual  variation  in 
these  and  similar  plants,  such  conclusions  need  verification  by  a  long 
series  of  examinations  before  they  can  be  accepted  as  final. 


MISCELLANEOUS   SPECIES. 


39 


The  quality  of  the  rubber  from  Apocynwn  is  said  by  Fox  to  be  much 
superior  to  milkweed  rubber,  as  will  be  noted  on  p.  54. 

TABLE  10. — Chemical  analyses  of  Apocynum  cannabinum  and  A.  androscemifolium. 


No. 

Place  of  collection. 

Date  of 
collection. 

Part  of  plant 
analyzed. 

Acetone 
extract. 

Rubber 
(benzene 
extract)  . 

328 

M  innehaha,  Colo.  *  

Aug.     1 

Leaves  

p.  ct. 
20.7 

p.  ct. 
1.2 

Stems     

23.7 
12  4 

0.62 
0.68 

384 

Fort  Scott,  Kans.2  

.  .  do 

Leaves   

6  5 

0  79 

Stems     

6.5 

1  0 

888 

Wichita,  Kans.  2  

Sept.    2 

Leaves       

11  0 

1  6 

Whole  

11.0 

1.3 

943 

Mount  Diablo,  Calif.  2  

July     9 

Leaves   

9.1 

2.2 

Stems     

7.2 

1.1 

1984 

Salt  Lake  City,  Utah  1.  .  . 

Sept.    1 

Leaves   

9  8 

0  70 

Stem£  

6.6 

0.22 

1039 

Lincoln,  Nebr.  2  

Oct.    15 

5  1 

Stems     

6  4 

4.5 
4.6 
4.6 
0.8 

6.0 

1.0 

1  A.  androssemifolium. 


a  A.  cannabinum . 


VI.  MISCELLANEOUS  SPECIES. 

In  addition  to  the  10  species  reported  upon  in  the  preceding  pages 
there  are  6  which,  while  not  of  such  promise  as  these,  yet  will  need  to  be 
considered  in  connection  with  any  study  of  the  native  rubber  plants 
of  North  America.  In  all  cases  the  number  of  samples  analyzed  is 
BO  small  that  further  examinations  should  be  made.  Quite  possibly 
some  of  the  species  placed  in  this  second  list  will  be  found  on  further 
study  to  excel  those  of  the  preceding  group.  All  but  the  last  two  are 
members  of  the  Asclepiadacese,  or  milkweed  family. 

Acerates  auriculata. — This  is  a  green  and  nearly  glabrous  plant  with 
the  aspect  of  an  Asclepias.  The  stems  are  3  feet  or  less  high  and  the 
leaves  are  narrowly  linear  or  filiform,  so  that  the  amount  of  rubber- 
carrying  herbage  that  could  be  produced  on  a  given  area  of  land  would 
not  be  so  great  as  that  from  the  true  milkweeds.  The  species  ranges 
from  Nebraska  and  Colorado  to  western  Texas  and  New  Mexico,  and 
grows  in  dry,  stony,  or  sandy  soil.  A  single  collection  (389)  from 
Denver,  Colorado,  yielded  on  analysis  2.9  per  cent  of  rubber  from 
the  leaves  and  0.5  per  cent  from  the  stems.  Another  species,  the 
green  milkweed  (A.  viridiflora) ,  with  much  more  copious  foliage,  is 
reported  upon  in  table  12. 

Asclepias  eriocarpa. — The  robust  habit,  leafiness,  and  ecologic  re- 
quirements of  this  species  are  such  as  to  render  it  especially  suited 


40 


RUBBER-CONTENT   OF   NORTH   AMERICAN   PLANTS. 


to  rubber  culture.  The  few  samples  thus  far  analyzed  furnish  a  very 
insufficient  basis  for  a  judgment  as  to  the  amount  of  rubber  that  it  may 
be  expected  to  yield.  The  stems  grow  in  clumps  to  a  height  of  2  to 
3  feet  and  are  densely  clothed  with  broadly  oblong,  hoary-tomentose 
leaves  in  whorls  of  3  or  4  at  each  node.  The  species  is  an  inhabitant 
of  poor  soils,  often  growing  in  places  too  stony  or  dry  for  other  than 
a  low  growth  of  herbaceous  plants,  and  is  limited  in  its  distribution  to 
California,  from  the  upper  end  of  the  Sacramento  Valley  south  to 
San  Diego  County,  growing  in  the  hottest  valleys  and  on  exposed 
foothill  slopes,  but  not  upon  the  desert.  Although  often  crowded 
out  into  poor  soils,  it  responds  readily  to  better  treatment,  as  is  evident 
by  the  exceptionally  vigorous  growth  made  when  it  is  occasionally 
permitted  to  develop  in  young  orchards  or  cultivated  fields.  A  plant 
of  this  sort  (915),  gathered  at  Atascadero,  in  the  south  Coast  Ranges, 
October  15,  1919,  was  found  to  contain  2.2  per  cent  of  rubber  in  the 
leaves  and  0.7  per  cent  in  the  stems.  The  parts  analyzed  were  from 
second  growth,  the  original  spring  growth  having  been  cut  earlier 
in  the  season.  The  leaves  of  a  plant  (939)  gathered  at  Auburn,  in  the 
Sierra  Nevada  foothills,  on  June  19,  1920,  yielded  1.3  per  cent  of 
rubber.  Another  sample  (944)  from  Cortiing,  in  the  northern  part  of 
the  Sacramento  Valley,  gathered  July  12,  1920,  carried  2.4  per  cent 
in  its  leaves  and  0.5  per  cent  in  the  stems. 

TABLE  11. — Chemical  analyses  of  Asdepiodora  decumbens. 


No. 

Place  of  collection. 

Date   of 
collection. 

Part  of  plant 
analyzed. 

Acetone 
extract. 

Rubber 
(benzene 
extract). 

339 

Trinidad   Colo 

Aug.  15 

Leaves  

p.  ct. 
9  5 

p.  ct. 
1  2 

372 

Near  Manitou,  Colo.     .  .  . 

July   14 

Whole  

9  2 

1.3 

882 

Pike's  Peak,  Colo         .... 

Aug.  31 

Whole  

8  0 

0.8 

983 

Near  Salt  Lake  City  Utah 

Sept.    1 

Leaves  

7  7 

1  7 

Stems     

9  1 

0  4 

985 

do     

...do  

Leaves  

8  5 

1  0 

Stems  

10  0 

0  5 

Asclepiodora  decumbens. — The  stems  of  this  milkweed  are  1  to  2 
feet  long  and  spreading  or  decumbent.  They  grow  in  clumps  from 
deep,  perennial  roots.  The  leaves  are  alternate,  lanceolate,  0.25  to 
nearly  1  inch  wide,  and  nearly  glabrous.  The  plants  resemble  some 
of  the  species  of  Asclepias  in  general  appearance  and  could  probably 
be  handled  much  like  A.  latifolia  or  A.  calif ornica,  but  they  differ 
from  all  of  the  true  milkweeds  in  the  entirely  alternate  leaves  and  in 
the  flowers,  the  corona-hoods  of  which  are  prominently  crested  within 
instead  of  being  provided  with  hornsi  In  the  bast  fibers  and  the  down 
on  the  seeds  they  are  quite  similar  to  Asclepias.  The  distribution  is 
from  Kansas  west  to  Utah  and  south  to  Texas,  Arizona,  and  Mexico, 


MISCELLANEOUS   SPECIES.  41 

on  dry  slopes  and  mesas.  The  analyses  are  not  encouraging,  but  are 
of  interest  in  that  they  demonstrate  positively  the  presence  of  rubber 
in  this  genus  of  plants  and  show  that  the  leaves  carry  a  higher  per- 
centage than  the  stems.  Perhaps  the  most  significant  result  is  the 
finding  of  1.3  per  cent  in  a  mixture  consisting  of  a  stem  with  all  of  its 
leaves  (372). 

Cryptostegia  grandiflora. — This  is  a  native  of  the  Old  World, 
probably  of  Indian  origin,  and  as  such  scarcely  falls  within  the  scope 
of  the  present  survey.  A  sample  was  sent  by  a  correspondent  of  Los 
Angeles,  California,  with  the  statement  that  it  came  from  Lower 
California.  If  this  is  correct,  the  sample  was  from  a  cultivated  speci- 
men. The  plant  has  been  used  in  India  for  its  caoutchouc,  but  not  to 
any  considerable  extent.  It  is  a  woody  climber  that  is  often  grown  in 
greenhouses  for  its  showy  blue  flowers  and  endures  the  out-of-doors 
temperature  of  Florida.  The  sample  mentioned  (935)  was  found  to 
carry  1  per  cent  of  rubber  in  the  stems  and  5.1  per  cent  in  the  foliage. 
These  determinations  are  for  pure  rubber  and  are  computed  on  the 
basis  of  dry  weight.  The  product  so  obtained  is  shiny,  very  elastic, 
and  may  be  pulled  out  into  threads  an  inch  long. 

Jatropha  cardiophylla. — The  jatrophas  are  shrubs  and  trees  of  the 
warmer  parts  of  both  hemispheres.  The  genus,  which  belongs  to  the 
Euphoriacese,  or  spurge  family,  is  well  represented  in  Central  America 
and  Mexico,  a  few  species  occurring  also  in  the  southwestern  part  of 
the  United  States.  J.  cardiophylla  is  a  shrub  1  to  4  feet  high  which 
grows  on  the  very  dry  hills  of  southern  Arizona  and  northern  Sonora. 
It  seems  to  be  especially  common  on  the  lower  slopes  of  the  Santa 
Catalina  Mountains.  The  leaves,  which  appear  only  after  the  summer 
rains,  are  broadly  ovate  or  rhombic,  with  a  truncate  or  cordate  base, 
and  are  0.5  to  1.5  inches  wide.  The  plants  readily  produce  new  stems 
when  pollarded  and  are  easily  propagated  by  cuttings.  A  collection 
(993)  made  on  Tumamoc  Hill,  Tucson,  Arizona,  February  21,  1920, 
was  found  on  analysis  to  carry  3  per  cent  of  rubber  in  the  stems. 
Further  examinations  should  be  made  of  this  species,  as  well  as  of  all 
others  of  the  Southwestern  States. 

Hymenoxys  floribunda  utilis  (Colorado  rubber  plant,  or  Pinguay). — 
This  composite  herb  has  been  already  discussed  (p.  9).  Two  collec- 
tions have  been  analyzed  during  the  course  of  the  studies.  One  of 
these  (931),  from  Las  Vegas,  New  Mexico,  was  divided  into  two 
parts.  That  portion  consisting  of  the  root  and  the  old  stem-bases 
contained  3.6  per  cent  of  rubber.  The  other  portion,  consisting  of 
the  main  stems  and  leaves,  contained  0.84  per  cent.  The  other  col- 
lection (947)  was  made  at  Buena  Vista,  Colorado,  August  1,  1920. 
In  this  the  whole  plant  was  ground  up  and  the  mixture  of  root,  stem, 
and  leaf  yielded  0.9  per  cent  of  rubber.  All  of  these  analyses  were  by 


42  RUBBER-CONTENT   OF   NORTH   AMERICAN   PLANTS. 

the  acetone-benzene  method  as  already  described,  so  that  the  per- 
centages are  taken  to  represent  the  proportion  of  pure  rubber  in  the 
samples.  This  perhaps  accounts  for  the  low  percentages  as  compared 
with  those  reported  by  others. 

VII.  SPECIES  WITH  VERY  LOW  RUBBER-CONTENT. 

In  the  course  of  the  investigation  it  was  found  that,  while  perhaps 
all  latex-bearing  plants  contained  rubber,  it  occurs  in  most  of  the 
species  in  such  small  quantities  as  to  be  negligible.  However,  it  is 
quite  probable  that  further  studies  will  demonstrate  considerably 
higher  percentages  in  the  individual  plants  of  some  of  the  species  here 
reported  as  containing  but  small  amounts.  This  assumption  is  based 
on  the  fact  that  rubber-content  varies  with  ecologic  conditions  and 
with  the  hereditary  strain  or  race.  It  is  also  probable  that  the  per- 
centage content  of  these  low-grade  species  could  be  increased  by 
bringing  them  under  cultivation,  where  they  could  be  subjected  to  a 
considerable  variety  of  cultural  methods,  such,  for  example,  as  changes 
in  the  water  relation,  and  where  they  could  be  improved  through 
selection  and  breeding.  However  this  may  be,  it  seems  unnecessary 
to  do  more  than  to  give  the  results  of  our  analyses  at  the  present  time, 
and  so  in  table  12,  which  refers  for  the  most  part  to  very  poor  species, 
will  be  found  a  few  in  which  the  rubber-content  is  sufficiently  high  to 
be  of  more  than  passing  interest. 

All  of  the  species  listed  in  table  12  are  latex  plants,  with  the  excep- 
tion of  Aster  spinosus,  Hymenoxys  odorata,  Pyrrhopappus  multicaulis, 
and  Silphium  latiniatum.  The  discovery  of  rubber  in  these  species  is 
especially  noteworthy  in  that  its  presence  has  not  been  heretofore 
known  in  any  of  the  genera  to  which  they  belong,  with  the  exception 
of  Hymenoxys.  This  is  of  some  biologic  although  not  of  much  com- 
mercial interest.  There  is,  of  course,  a  possibility  that  the  benzene 
extract  reported  for  these  as  "rubber"  may,  in  some  instances,  be  some 
other  substance,  something  that  comes  out  with  the  benzene  extract 
through  errors  in  methods  or  manipulation.  In  each  case,  however, 
the  results  have  been  checked  by  one  or  more  duplicate  analyses  and 
the  percentages  are  sufficiently  high  and  sufficiently  in  agreement  to 
justify  a  fair  degree  of  confidence  in  the  report. 

Table  12  includes  some  latex  plants  in  which  the  benzene  extract  is 
so  low,  1  per  cent  or  less,  that  it  can  not  be  said  with  certainty  that 
rubber  is  present.  Yet  since  every  species  with  a  latex  is  of  interest 
as  a  possible  carrier  of  rubber,  all  of  these  are  here  listed  for  comparison, 
regardless  of  the  percentage  found  on  analysis. 

In  cases  where  the  part  of  the  plant  analyzed  is  reported  as  "whole" 
this  is  to  be  taken  as  exclusive  of  root  and  flowers;  in  other  words,  the 
stems  and  foliage  were  ground  up  and  analyzed  together.  All  per- 
centages are  computed  upon  the  basis  of  absolutely  dry  weight  of  the 
material  analyzed. 


SPECIES   WITH   VERY   LOW   RUBBER-CONTENT. 


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46  RUBBER-CONTENT   OF  NORTH  AMERICAN  PLANTS. 

VIII.  SPECIES  EXAMINED  WITH  NEGATIVE  RESULTS. 

In  the  attempt  to  discover  as  many  rubber-bearing  species  as  possible 
many  sorts  were  examined  on  the  mere  chance  that  rubber  might  be 
present.  In  a  few  cases  it  was  unexpectedly  found  in  non-latex 
plants,  as  indicated  in  the  preceding  table.  The  remaining  species, 
those  in  which  no  rubber  could  be  detected  with  certainty,  are  enum- 
erated below.  All  were  subjected  to  a  chemical  analysis,  but  in  no 
case  was  more  than  a  fraction  of  1  per  cent  of  benzene  extract  obtained. 
Such  small  amounts  are  almost  certainly  some  other  substance  than 
rubber  when  they  occur  in  non-latex  plants.  The  locality  mentioned 
for  each  species  is  the  place  of  collection  of  the  sample. 

A.  COMPOSITE. 

876.  Achillsea  millefolium,  Pike's  Peak,  Colorado. 
393.  Ambrosia  artemisiifolia,  Manitou,  Colorado. 

395.  Ambrosia  trifida,  Manitou,  Colorado. 

873.  Anaphalis  margaritacea,  Mount  Manitou,  Colorado. 

396.  Artemisia  campestris,  Manitou,  Colorado. 

877.  Aster  bigelowi,  Pike's  Peak,  Colorado. . 

865.  Bahia  dissecta,  Manitou,  Colorado. 

1107.  Baileya  multiradiata,  Santa  Rita  Reserve,  Arizona. 
906.  Balsamorhiza  deltoidea,  near  La  Grande,  Oregon. 

1040.  Balsamorhiza  deltoidea,  Ogderi,  Utah. 
904.  Bidens  frondosa,  Twin  Falls,  Idaho. 

878.  Brickellia  wrighti,  Canyon  City,  Colorado. 

908.  Chrysanthemum  balsamita,  Grant's  Pass,  Oregon. 

868.  Chrysopsis  villosa,  Manitou,  Colorado. 

946.  Chrysothamnus  howardi,  Buena  Vista,  Colorado. 

809.  Dysodia  papposa,  Northern  Texas. 
1104.  Encelia  farinosa,  Tucson,  Arizona. 

326.  Eriophyllum  lanatum  integrifolium,  Eastern  Colorado. 
875.  Gaillardia  aristata,  Royal  Gorge,  Colorado. 

819.  Gutierrezia  texana,  Texas. 

867.  Gymnolomia  multiflora,  Manitou,  Colorado. 

883.  Haplopappus  parryi,  Pike's  Peak,  Colorado. 

864.  Haplopappus  spinulosus,  Colorado  Springs,  Colorado. 

929.  Helianthus  ciliarus,  near  El  Paso,  Texas. 

948.  Hymenopappus  filifolius,  Buena  Vista,  Colorado. 

392.  Iva  xanthifolia,  Manitou,  Colorado. 

866.  Kuhnia  eupatorioides,  Manitou,  Colorado. 
862.  Liatris  punctata,  Colorado  Springs,  Colorado. 

852.  Parthenium  incanum,  San  Andreas  Mountains,  New  Mexico. 

850.  Pectis  angustifolia,  Sonora,  Texas. 

810.  Pectis  papposa,  Western  Texas. 

901.  Pericome  caudata,  Pike's  Peak,  Colorado. 

1108.  Psilostrophe  cooperi,  Tucson,  Arizona.1 
830.  Senecio  filifolius,  Western  Texas. 

903.  Solidago  occidentalis,  Twin  Falls,  Idaho. 

861.  Thelesperma  trifida,  Colbrado  Springs,  Colorado. 

874.  Verbesina  dissecta,  Manitou,  Colorado. 

820.  Vernonia  marginata,  Northern  Texas. 

851.  Zinnia  grandiflora,  Las  Cruces,  New  Mexico. 

1  A  sample  of  Psilostrophe  cooperi  from  Las  Cruces,  New  Mexico  (No.  805,  September,  1919) 
yielded  1.4  per  cent  of  benzene  extract,  indicating  that  rubber  is  possibly  present  in  this  species. 


VARIATION  IN  RUBBER-CONTENT  OF  ASCLEPIAS  AND  APOCYNUM.    47 

B.  MISCELLANEOUS  FAMILIES. 

305.  Argemone  platyceras  (Papaveraceae),  Manitou,  Colorado. 
938.  Aristolochia  calif ornica  (Aristolochiaceae),  Newcastle, 

California. 

375.  Comandra  umbellata  (Santalaceae) ,  Colorado  Springs,  Colorado. 
1117.  Convolvulus  occidentals  (Convolvulaceae),  La  Jolla,  Califor- 
nia. 
1111.  Convolvulus  soldanella  (Convolvulaceae),  La  Jolla,  California.2 

362.  Croton  calif  ornica  (Euphorbiacese),  Brentwood,  California. 
806.  Croton  corymbulosus  (Euphorbiaceae),  Las  Cruces,  New  Mex- 
ico. 

828.  Croton  corymbulosus  (Euphorbiaceae),  Sonora,  Texas. 

847.  Croton  texensis  (Euphorbiacese),  Sonora,  Texas. 

363.  Eremocarpus  setigerus  (Euphrobiacese),  Antioch,  California. 
1109.  Fouquieria  splendens  (Fouquieriaceae),  Tucson,  Arizona. 

390.  Gaura  parviflora  (Onagraceae),  Manitou,  Colorado. 

907.  Hypericum  perforatum  (Hypericaceae),  Grant's  Pass,  Oregon. 

812.  Maclura  pomifera  (Moraceae),  Manhattan,  Kansas. 

825.  Microrhamnus  ericoides  (Rhamnaceae),  Western  Texas. 

378.  Oenothera  serrulata   (Onagraceae),   near  Colorado    Springs, 

Colorado. 

989.  Olea  europaea  (Oleaceae),  Tucson,  Arizona  (cult.). 
811.  Othake  sphacelatum  (Compositae),  near  Guymon,  Oklahoma. 
863.  Psoralea  tenuiflora  (Leguminosae),  Colorado  Springs,  Colorado. 

379.  Smilax  herbacea  (Smilacaceae),  Eastern  Colorado. 
1130.  Stillingia  annua  (Euphorbiaceae),  Glamis,  California. 

803.  Stillingia  silvatica  (Euphorbiaceae),  Northern  Texas. 

848.  Stillingia  torreyana  (Euphorbiaceae),  Northern  Texas. 
881.  Tragia  ramosa  (Euphorbiaceae),  Royal  Gorge,  Colorado. 

IX.  VARIATION  IN  THE  RUBBER-CONTENT  OF  ASCLEPIAS 

AND  APOCYNUM. 

Distribution  of  rubber  in  the  plant. — Since  the  rubber  both  in 
Asclepias  and  in  Apocynum  is  carried  by  the  latex,  it  is  not  surprising  to 
find  that  the  highest  percentages  are  obtained  from  those  plant  parts 
in  which  the  laticiferous  tissue  is  well  developed.  As  will  be  seen  from 
an  examination  of  the  tables  of  analyses,  the  foliage  nearly  always 
carries  a  much  higher  percentage  than  any  other  part.  In  all  but  one 
species  the  amount  carried  by  the  sterna  is  so  slight,  usually  less  than 
1  per  cent  of  their  dry  weight,  that  its  recovery  in  commercial  rubber 
manufacture  is  perhaps  not  to  be  considered.  However,  the  occa- 
sional appearance  of  a  plant  in  which  the  stem  carries  a  fair  amount 
of  rubber  gives  rise  to  the  suggestion  that  improved  strains  might  be 
developed  in  which  stems  as  well  as  leaves  could  be  utilized.  Such  a 
plant  is  one  of  Asclepias  mexicana  (No.  917)  which  yielded  on  analysis 

2  A  second  sample  of  Convolvulus  soldanella,  from  the  sand-dunes  at  La  Jolla  (No.  1114,  April 
3,  1920)  yielded  1.5  per  cent  of  benzene  extract  from  the  stems,  indicating  that  rubber  may 
possibly  be  present  in  small  amount. 


48  RUBBER-CONTENT   OF  NORTH   AMERICAN   PLANTS. 

4.4  per  cent  from  the  leaves  and  2.3  per  cent  from  the  stems.  This 
plant  was  growing  in  alkaline  soil,  which  fact  may  have  influenced 
the  formation  of  rubber  in  its  stems  or  perhaps  have  caused  its  trans- 
location  from  the  leaves.  It  is  to  be  noted  that  in  A.  mexicana  the 
stems  are  green  and  doubtless  share  with  the  foliage  in  the  photo- 
synthetic  activities  of  the  plant.  It  is  probable  that  a  species  of  this 
sort  would  respond  more  readily  to  experiments  designed  to  increase 
the  rubber-content  of  the  stalks  than  would  one  in  which  the  stems 
played  little  or  no  part  in  photosynthesis. 

The  exceptional  species  referred  to  above,  in  which  the  stems  carry 
more  than  a  negligible  percentage  of  rubber,  is  Asdepias  subulata,  a 
milkweed  of  the  southwestern  deserts  and  Lower  California.  In  this 
plant  the  leaves  are  so  few  and  small  that  it  has  not  been  possible  thus 
far  to  assemble  a  sample  for  analysis,  but  the  stems  themselves  take 
over  the  r61e  of  leaves  to  a  considerable  extent  and  in  consequence 
are  found  to  carry  considerable  amounts  of  rubber,  in  some  specimens 
as  high  as  6  per  cent.  It  is  thus  again  suggested  that  the  formation 
of  rubber  is  closely  connected  with  the  photosynthetic  activity  of  the 
plant.  This  in  turn  lends  support  to  the  view  that  the  formation  of 
caoutchouc  is  closely  associated  with  that  of  carbohydrates.  In  this 
connection  it  should  be  noted  that  the  bark  of  the  stem  in  A.  subulata 
carries  much  more  rubber  than  the  stele  itself  (No.  957  of  table  1). 
This  suggests  that  there  is  a  prompt  transformation  of  the  carbo- 
hydrates if  the  caoutchouc  is  a  conversion-product  of  the  sugars,  as 
investigators  of  the  chemistry  of  rubber  formation  are  now  inclined 
to  believe  (Harries,  1905,  1919). 

The  almost  total  absence  of  rubber  in  the  old  woody  parts,  and 
especially  in  the  roots,  is  expected  in  latex  plants.  That  this  holds  for 
Asdepias  is  shown  by  a  number  of  analyses  of  these  parts  (for  example, 
No.  1015  of  table  1,  and  No.  304  of  table  9). 

Variation  due  to  heredity  and  to  environment. — During  the  three 
years'  time  devoted  to  the  studies  here  described,  much  attention  has 
been  given  to  the  extent  and  cause  of  the  variation  in  rubber-content 
of  the  species  of  Asdepias.  With  all  of  the  data  now  assembled,  the 
only  positive  statement  that  can  be  made  is  that  there  is  a  wide  fluc- 
tuation both  between  the  16  species  examined  and  within  each  of  the 
several  species  studied  in  detail.  In  Asdepias  sullivanti,  which  has 
received  more  attention  than  the  others,  the  content  was  found  to 
range  from  1.2  to  8.1  per  cent  in  the  leaves  and  0.2  to  0.9  (in  one  anom- 
alous case  8.2)  per  cent  in  the  stems.  In  case  an  attempt  is  made 
to  improve  the  plants  for  commercial  purposes,  it  will  be  important  to 
determine  whether  such  variations  are  due  to  differences  in  the  herit- 
able properties  or  to  environmental  causes.  A  start  has  been  made 
on  this  problem,  but  it  is  so  baffling  that  no  positive  conclusion  has 
been  reached.  Some  of  the  results  will  be  indicated  a  little  farther 


VARIATION  IN  RUBBER-CONTENT  OF  ASCLEPIAS  AND  APOCYNUM.  49 


on,  but  it  may  now  be  said  that  probably  both  heredity  and  environ- 
ment are  concerned;  that  is,  that  each  species  is  composed  of  many 
strains,  each  with  a  different  rubber-producing  capacity,  and  that  at 
the  same  time  the  amount  of  rubber  present  in  plants  of  each  strain 
is  modified  by  the  local  environment.  If  this  assumption  is  correct, 
the  chances  of  improvement  through  selection  and  breeding,  and  also 
through  the  manipulation  of  the  environment,  are  greatly  enhanced. 
There  is  a  wide  individual  variation  in  plants  growing  under  uniform 
conditions,  as  is  indicated,  although  not  finally  proven,  by  detailed 
studies  made  on  several  species.  For  example,  4  plants  of  Asclepias 
sullivanti  growing  near  Lincoln,  Nebraska,  in  what  appeared  to  be 
uniform  soil  and  only  a  few  feet  from  one  another,  were  all  gathered 
at  the  same  time  (August  28,  1920)  and  analyzed  separately,  with  the 
following  results: 


TABLE  13.- 


-Variation  in  rubber-content  of  plants  of  Asclepias  sullivanti 
growing  under  apparently  uniform  conditions. 


Stems. 

Leaves. 

No. 

Acetone 
extract. 

Rubber 
(benzene 
extract). 

Acetone 
extract. 

Rubber 
(benzene 
extract)  . 

p.  ct. 

p.  ct. 

p.  ct. 

p.  ct. 

1016 

6.6 

0.5 

9.5 

5.4 

6.0 

0.4 

9.3 

5.3 

1017 

6.8 

0.2 

8.1 

3.1 

6.5 

0.2 

8.9 

2.9 

1018 

8.0 

1.0 

8.5 

1.2 

7.5 

0.8 

8.5 

1.1 

1019 

7.0 

0.9 

7.0 

2.9 

7.0 

0.8 

7.2 

3.0 

The  extreme  variation  from  1.1  to  5.4  per  cent  in  the  leaves,  as  indi- 
cated in  this  table,  can  scarcely  be  attributed  to  what  slight  difference 
there  might  have  been  between  the  environments  of  the  two  plants. 
Similar  evidence  is  deduced  from  the  analyses  of  3  plants  of  Asclepias 
subulata  gathered  at  Sentinel,  Arizona.  The  rubber-content  of  the 
stems  of  these  plants,  the  foliage  being  so  sparse  as  to  be  negligible, 
was  found  to  be  2.6,  3.5,  and  4.4  per  cent  respectively.  In  this  case 
the  differences  are  conceivably  due  to  slight  inequalities  in  the  soil 
surface  and  hence  in  the  water  relation. 

A  third  series  of  samples  was  taken  to  test  this  point,  this  time  of 
Asclepias  mexicana.  Three  plants  (910,  911,  and  912)  were  selected 
near  Mount  Diablo,  California,  all  apparently  alike  and  all  from 
ivithin  a  foot  from  one  another  in  a  field  where  the  herbaceous  vegeta- 
tion exhibited  great  uniformity  in  its  growth.  The  leaves  of  these 
plants  were  analyzed  and  found  to  contain  in  one  case  2.8  per  cent, 


50  RUBBER-CONTENT   OF   NORTH   AMERICAN   PLANTS. 

in  another  3.2  per  cent,  and  in  the  third  4.1  per  cent  of  rubber.  In 
no  species  have  approximately  equal  amounts  been  found  in  more  than 
2  plants  from  a  single  small  area. 

The  evidence  thus  far  available  seems,  therefore,  to  point  to  the 
presence  of  innumerable  strains  within  at  least  some  species  of  Asclepias, 
the  rubber-content  varying  with  these  strains,  as  well  as  with  changes 
in  the  environment. 

The  influence  of  soil,  moisture,  and  other  external  conditions  has 
not  been  studied  in  detail,  but  there  is  some  indication  that  the  presence 
of  alkali  in  the  soil  increases  the  rubber-content  of  Asclepias  mexicana. 
On  the  other  hand,  there  is  no  evidence  that  poor  or  dry  soil  or  defi- 
ciency in  rainfall  increases  the  amount.  In  fact,  some  of  the  best 
rubber-producing  species,  such  as  Asclepias  sullivanti,  grow  only  on 
rich  and  moist  bottom-lands.  Within  single  species  the  plants  from 
poor  soil  average  about  the  same  as  those  from  good  soil,  but  with 
some  indication  that  especially  robust  plants  are  likely  to  carry  rather 
high  percentages  of  rubber,  this  being  associated,  perhaps,  with  their 
increased  photosynthetic  activity. 

Seasonal  variation. — Detailed  studies  were  carried  out  on  several 
species  in  order  to  determine  the  presence  or  absence  of  a  correlation 
between  the  age  of  the  plant  and  its  rubber-content.  It  was  thought 
that  such  correlation,  if  it  existed,  would  throw  some  light  upon  the 
perplexing  question  of  the  role  that  rubber  plays  in  the  economy  of 
the  plant.  The  effect  of  other  factors,  however,  is  too  great  to  permit 
the  drawing  of  any  definite  conclusions.  Aside  from  such  obvious 
modifying  influences  as  changes  in  temperature  and  moisture,  the 
translocation  of  rubber  within  the  plant  must  be  allowed  for,  and  more 
important  than  all,  the  wide  extent  of  individual  variation.  It  is 
obvious  that  analyses  made  from  leaves  of  a  single  plant  but  taken 
on  different  dates  would  yield  data  of  very  doubtful  value,  since  there 
is  no  evidence  that  leaves  in  different  positions  carry  equal  percentages 
even  at  the  same  time.  Moreover,  the  removal  of  a  sufficient  number 
of  leaves  to  permit  of  analysis  would  almost  certainly  modify  the 
process  of  rubber  formation  in  the  rest  of  the  plant.  With  these 
difficulties  in  view,  seasonal  studies  on  several  species  were  inaugurated. 
There  is  a  lack  of  concordance  in  the  results,  but  the  indications  are 
that  both  the  total  amount  and  the  percentage  increases  as  the  herbage 
matures,  but  that  they  again  fall  off  as  the  parts  approach  senility, 
or,  in  the  case  of  stems,  as  lignification  sets  in. 

An  experiment,  the  results  of  which  seem  to  controvert  the  theory 
that  the  percentage  increases  with  maturity,  will  be  first  described. 
Several  plots  of  Asclepias  sullivanti  were  selected  near  Lincoln, 
Nebraska,  and  samples  from  them  were  taken  at  intervals  of  approxi- 
mately 8  days  each.  Each  lot  was  chosen  because  of  the  apparent 


VARIATIONS  IN  RUBBER-CONTENT  OF  ASCLEPIAS  AND  APOCYNUM.  51 


uniformity  of  the  plants,  and  5  specimens  were  taken  from  each  lot 
at  the  close  of  each  period.  It  was  thought  that  by  thoroughly  mixing 
the  dried  and  ground  material  of  5  plants  a  reliable  average  for  the 
whole  lot  would  be  obtained.  The  results  are  shown  in  table  14. 

TABLE  14. — Seasonal  variation  in  the  rubber-content  of  Asclepias  sullivanti  at  Lincoln, 

Nebraska. 


Stems. 

Leaves. 

No. 

Date. 

Precipita- 
tion for 
preceding 
period. 

Mean 
tempera- 
ture for 
preceding 
period. 

Acetone 
extract. 

Rubber 
(benzene 
extract)  . 

Acetone 
extract. 

Rubber 
(benzene 
extract). 

p.  ct. 

p.  ct. 

p.  ct. 

p.  ct. 

inches. 

o  p 

Lot  1: 

1020 

Aug.  28 

'    4.9 

0.4 

7.8 

3.6 

0.99 

66 

4.3 

0.4 

8.5 

3.6 

1022 

Sept.    6 

6.0 

0.2 

6.8 

4.1 

1.31 

66 

6.8 

0.2 

7.8 

4.0 

1025 

Sept.  14 

6.3 

0.4 

10.7 

2.6 

0.34 

70 

6.4 

0.4 

11.0 

2.7 

1028 

Sept.  22 

4.0 

0.2 

8.0 

3.0 

0.00 

76 

4.5 

0.2 

7.8 

2.9 

Lot  2: 

Aug.  28 

0.6 

3.1 

0.99 

66 

1023 

Sept.    6 

10.0 

0.4 

10.0 

2.6 

1.31 

66 

8.5 

0.5 

11.1 

2.7 

1026 

Sept.  14 

2.1 

0.3 

10.0 

3.7 

0.39 

70 

2.5 

0.3 

9.7 

3.5 

1029 

Sept.  22 

4.3 

0.5 

9.0 

4.3 

0.00 

76 

4.8 

0.6 

11.0 

4.0 

Lot  3: 

1021 

Aug.  28 

6.1 

0.2 

7.8 

4.0 

0.99 

66 

8.2 

0.1 

7.6 

4.0 

1024 

Sept.    6 

1.3 

0.3 

8.2 

2.0 

1.31 

66 

1.0 

0.2 

8.2 

2.0 

1027 

Sept.  14 

6.2 

0.2 

11.8 

4.2 

0.34 

70 

6.0 

0.2 

11.6 

4.1 

1030 

Sept.  22 

1.7 

0.1 

8.1 

4.2 

0*00 

76 

2.0 

0.1 

8.4 

4.2 

In  considering  the  results  set  down  in  table  14,  only  the  rubber-con- 
tent of  the  leaves  need  be  taken  into  account,  since  the  amount  in 
the  stems  is  so  slight  that  the  differences  are  not  significant.  For 
lot  2  it  will  be  noticed  that,  with  the  exception  of  the  first  period,  the 
rubber-content  gradually  increased  as  the  season  advanced.  This  is 
in  agreement  with  results  often  but  not  always  obtained  in  other 
species.  The  analyses  of  lots  1  and  3,  however,  entirely  negative 
this  conclusion  and  indicate  an  irregular  fluctuation  difficult  to  account 
for,  except  on  the  basis  of  some  unknown  factor  or  on  that  of  individual 
variation,  which,  as  shown  in  table  13,  is  so  great  that  it  is  not  wise  to 
draw  conclusions  unless  a  very  large  number  of  plants  are  included  in 
each  analysis.  Therefore  the  principal  value  of  table  14  is  to  indicate 


52  RUBBER-CONTENT   OF   NORTH  AMERICAN   PLANTS. 

the  lack  of  concomitance  in  the  various  lots  and  thus  to  warn  against 
the  danger  of  explaining  seasonal  variation  on  the  basis  of  a  limited 
number  of  analyses  and  without  taking  all  possible  factors  into  account. 

With  due  regard  to  the  warning  just  given,  the  results  of  an  experi- 
ment on  Asclepias  mexicana  may  now  be  considered.  A  uniform 
growth  of  this  plant  on  an  area  a  few  meters  square  was  located  near 
Mount  Diablo,  California.  A  swath  was  cut  across  the  middle  of  this 
plot  on  July  9,  1920.  The  leaves  at  that  time  carried  3.7  per  cent. 
The  growth  along  both  sides  of  this  strip  was  cut  on  October  12  of  the 
same  year,  when  it  was  found  that  the  percentage  had  increased  to  4.6. 
The  content  of  the  stems  had  dropped  during  this  interval  from  1  to  0.8 
per  cent,  but  in  view  of  the  small  amount  of  rubber  in  the  stems  this 
slight  difference  is  not  considered  significant. 

Further  evidence  may  be  deduced  from  certain  of  the  tables  of 
analyses.  For  example,  in  table  3  several  instances  may  be  noted 
where  the  analysis  of  young  leaves  yielded  lower  percentages  than 
older  leaves  taken  from  farther  down  on  the  same  stem.  But  most 
convincing  of  all  are  comparisons  between  Nos.  1034,  1032,  and  1035, 
made  in  this  order  from  table  3.  Here  the  increase  with  age  is  per- 
fectly regular.  Also,  in  the  case  of  A.  subulata  (table  1),  the  green 
stems  are  found  to  carry  smaller  percentages  in  the  young  growth 
than  in  the  more  mature  but  not  woody  portions.  On  the  other  hand, 
the  percentage  of  rubber  in  very  old  leaves  and  stems  is  always  much 
reduced  (see  No.  919  of  table  4,  No.  342  of  table  5,  and  various  numbers 
of  table  1).  This  lends  support  to  the  theory  that  caoutchouc  may 
undergo  considerable  modification,  or  even  entirely  disappear,  after 
it  is  once  formed  in  the  plant. 

In  case  it  is  found  that  the  amount  of  rubber  increases  as  the  season 
advances,  as  seems  to  be  indicated  by  the  above  considerations,  it 
will  be  interesting  to  know  whether  this  is  due  to  an  increase  in  the 
volume  of  the  latex  or  to  an  increase  in  the  percentage  carried  by  the 
latex  itself.  The  only  evidence  at  hand  indicates  that  the  latter  may 
be  the  case.  A  number  of  plants  of  Asclepias  halli  growing  in  Ute 
Pass,  Colorado,  were  "bled"  on  July  17,  1919,  and  7  c.  c.  of  latex 
obtained  and  analyzed.  The  results  showed  but  4.7  per  cent  of 
rubber  in  the  water-free  latex.  A  similar  sample  was  obtained  from 
another  colony  of  plants  on  July  28,  1919.  This  was  found  to  carry 
21.0  per  cent  of  rubber  on  the  same  basis.  This  seems  to  indicate 
a  considerable  increase  in  the  richness  of  the  latex  for  A.  halli  as  the 
season  advances,  although  the  difference  may  be  due  to  individual 
variation. 

Variation  in  successive  crops. — When  the  stems  of  some  and  perhaps 
all  species  of  Asclepias  are  removed  by  mowing  or  cutting  in  any 
manner  near  the  base,  a  new  growth  of  shoots  will  spring  up.  Thus 


VARIATION  IN  RUBBER-CONTENT  OF  ASCLEPIAS  AND  APOCYNUM  53 

two  or  more  "crops"  may  be  obtained  in  a  season,  as  in  the  case  of  the 
well-known  alfalfa  or  lucerne.  Since  this  practice  would  greatly  in- 
crease the  tonnage  of  herbage,  its  use  in  case  the  plants  are  brought  under 
cultivation  is  at  once  suggested.  The  cutting  in  itself,  however,  might 
modify  the  percentage  content  of  succeeding  crops.  While  the  data 
at  hand  are  not  conclusive,  they  seem  to  indicate  that  the  second  crop 
would  carry  as  high  or  perhaps  a  higher  percentage  than  the  first,  but 
that  the  third  crop  might  show  a  considerable  decline,  at  least  in  plants 
grown  under  unfavorable  conditions. 

The  evidence  for  these  conclusions  is  as  follows:  A  grain  field  in 
Solano  County,  California,  was  infested  with  a  scattered  growth  of 
Asclepias  mexicana.  This  had  been  mowed  off  with  the  grain  in  the 
early  summer  of  1919,  before  a  study  of  its  rubber-content  could  be 
made.  By  November  13  of  the  same  year  the  new  shoots  had  reached 
maturity,  and  these  were  gathered  from  80  different  plants  (sample  No. 
923).  The  dried  and  mixed  leaves  analyzed  4.1  per  cent  of  rubber, 
which  is  much  higher  than  the  average  for  the  first  growth  of  this 
species  and  which  is  rarely  exceeded  even  by  single  plants.  Two 
other  collections  of  second-growth  material,  probably  from  one  root, 
were  made  near  Mount  Diablo,  California,  one  on  July  9,  1920  (No. 
941),  the  other  on  October  12  of  the  same  year  (No.  1037).  The 
leaves  of  these  analyzed  3.7  and  4.5  per  cent  respectively,  the  higher 
percentage  of  the  latter  corresponding  to  its  greater  degree  of  maturity. 
Again  the  percentage  of  the  first  crop  is  not  known,  but  these  returns 
for  the  second  crop  are  so  high  for  A.  mexicana  that  no  considerable 
falling-off  could  have  taken  place. 

The  only  evidence  that  the  percentage  may  drop  with  the  third  crop 
is  supplied  by  studies  of  the  above-mentioned  plot  near  Mount  Diablo, 
and  even  here  the  result  is  evidently  due  to  the  poorly  developed  con- 
dition of  the  third  crop  when  harvested.  The  stems  of  this  plot  are 
connected  by  horizontal  roots,  so  that  the  percentage  content  may 
be  assumed  as  the  same  for  all  at  any  one  time.  On  July  9,  1920,  a 
swath  was  cut  through  the  middle  of  the  second  growth  and  the 
leaves  and  stems  together  were  found  to  carry  2.1  per  cent  of  rubber 
(No.  941;  leaves  3.7  per  cent,  stems  1  per  cent).  By  October  12  it 
was  found  that  the  shoots  (stems  and  leaves)  not  cut  in  July  had 
increased  their  rubber-content  to  2.4  per  cent  (No.  1037;  leaves  4.5 
per  cent,  stems  0.8  per  cent),  whereas  the  third  crop  from  the  cut 
swath  yielded  only  1.5  per  cent  (No.  1038).  The  shoots  of  this  third 
crop  were  so  weak  and  slender  that  the  leaves  and  stems  were  not 
analyzed  separately.  It  is  possible  that  by  giving  the  plants  the 
proper  cultural  conditions,  and  by  so  timing  the  harvests  as  not 
seriously  to  weaken  the  roots,  a  third  crop  may  be  obtained  which 
would  be  as  rich  as  either  of  the  other  two.  The  effect  of  cutting 
upon  the  vigor  of  successive  crops  has  been  noted  only  under  the 


54  RUBBER-CONTENT  OF   NORTH  AMERICAN  PLANTS. 

unfavorable  conditions  obtaining  at  the  place  just  noted.  The  experi- 
ment should  be  repeated  on  some  of  the  robust  stands  in  the  alkaline 
districts  of  the  San  Joaquin  Valley  and  more  tune  given  the  third 
crop  to  fully  mature  its  stems  and  leaves. 

X.  PROPERTIES  OF  THE  RUBBERS. 

The  product  obtained  from  Asclepias  and  Apocynum  by  the  methods 
used  during  these  studies  is  true  rubber,  with  only  a  small  admixture 
of  resins,  fats,  or  other  substances  such  as  are  sometimes  obtained 
from  plants  and  called  by  this  name.  The  reasons  for  this  conclusion 
have  been  given  in  connection  with  the  discussion  of  the  chemical 
methods  employed  (p.  14).  Samples  have  not  been  prepared  by 
mechanical  processes,  but  the  accumulations  of  the  benzene  extract 
resulting  from  the  chemical  analyses  have  been  preserved  for  some  of 
the  species.  The  largest  accumulation  (4  grams)  is  from  Asclepias 
sullivanti.  This  sample  is  nearly  black  in  color  and  possesses  elasticity 
and  some  resilience.  It  emits  the  characteristic  odor  of  rubber  when 
it  is  burned.  During  the  4  months  since  its  preparation  the  surface 
has  undergone  slow  oxidation,  as  indicated  by  the  formation  of  resin- 
like  granules.  No  attempt  has  been  made  to  vulcanize  the  sample. 

The  properties  of.  a  rubber  prepared  from  some  unknown  species 
of  milkweed  have  been  reported  by  A.  T.  Saunders  (1910).  The  sam- 
ple, which  weighed  about  2  grams,  was  permitted  to  stand  for  10  years 
before  examination,  after  which  period  it  was  found  to  contain  but 
20  per  cent  of  hydrocarbon,  the  remainder  being  vegetable  matter  and 
resin-like  products  of  oxidation.  It  yielded  to  the  usual  solvents  of 
rubber,  resisted  the  same  reagents,  as  far  as  tested,  possessed  elasticity, 
and  was  susceptible  of  cure.  When  rolled  into  a  slender  thread  and 
subjected  to  the  acid  cure  it  was  extensible  to  five  times  its  length  and 
returned  sharply  upon  release.  Fox  (1911)  reported  caoutchouc 
obtained  from  Asclepias  syriaca  as  flabby,  lacking  in  strength  and  firm- 
ness, and  high  in  gravity.  It  responded  to  the  sulphur,  chloride,  and 
bromine  tests. 

The  few  and  rather  desultory  examinations  made  thus  far  of  Asclepias 
rubber  would  indicate  that  it  is  of  low  grade,  not  to  be  compared  with 
fine  Para,  with  imported  plantation  rubber,  nor  with  chrysil.  It  would 
doubtless  find  use  for  mixing  with  better  grades  and  in  those  manufac- 
tures where  great  elasticity  is  not  demanded.  It  is  possible,  however, 
that  other  species  will  yield  a  better  product  than  that  obtained  by  Fox 
from  A.  syriaca.  The  rubber  from  A.  sullivanti,  for  example,  seems 
to  be  better  than  the  sample  with  which  he  worked. 

Rubber  prepared  from  Apocynum  (Indian  hemp  and  dogbane)  is 
probably  superior  to  that  from  Asclepias.  Fox  (1912)  reports  his 
experience  in  the  preparation  of  rubber  from  A.  androscemifolium  as 
follows : 


PROPORTION  OF  THE  RUBBERS.  55 

"The  latex  of  Apocynum  differs  from  that  of  Asdepias  in  that  it  coagulates 
spontaneously,  even  if  it  is  kept  in  closed  containers.  The  spontaneously 
coagulated  latex  gives: 

Per  cent. 

Liquid  portion 67.58 

Cheese  (wet) «.  . . .     32.42 

'  'The  liquid  is  white,  slightly  acid,  and  [of  an]  acrid  odor.  This  liquid  failed  to 
coagulate  after  addition  of  more  acid.  Slight  excess  of  alkali  increased  its 
viscosity,  changed  its  color  from  white  to  brownish  yellow,  but  did  not 
coagulate  <^r  precipitate  it.  Boiling  had  no  effect.  Excess  of  acetone  gave 
a  finely  divided  precipitate,  the  particles  of  which  were  not  cohesive.  Evapo- 
ration of  the  mixture,  after  washing  with  water  and  treatment  with  boiling 
acetone,  gave  a  small  quantity  of  black,  soft  rubber,  destitute  of  strength. 
The  cheese  was  composed  of: 

Per  cent. 

Water 33 .46 

Rubber 3 .99 

Resin 62 .95 

"Working  up  this  cheese  of  the  plant  in  the  usual  manner  with  solvents, 
straining  through  gauze  to  remove  dirt,  evaporation  with  low  heat,  the  excess 
of  solvent,  adding  an  excess  of  precipitant,  washing  the  precipitant  and  dissi- 
pating the  precipitating  agent,  gave  a  good  grade  of  rubber. 

"The  rubber  obtained  in  this  manner  is  black,  firm,  not  tacky,  odorless,  and 
strong.  In  quality  it  is  much  better  than  the  product  obtained  from  its 
neighbor,  milkweed." 

In  drawing  conclusions  from  the  above  remarks  it  should  be  remem- 
bered that  no  thorough  test  has  as  yet  been  made  of  the  rubber  either 
of  Asdepias  or  of  Apocynum,  and  that  considerable  variation  may  be 
expected  in  the  rubbers  prepared  from  different  species.  The  collec- 
tion of  material  in  quantity  for  the  purpose  of  extracting  enough  of  the 
rubber  to  permit  of  extensive  tests  is  now  being  undertaken.  Several 
of  the  more  important  species  will  be  studied  in  this  manner. 


56  RUBBER-CONTENT   OF   NORTH  AMERICAN  PLANTS. 

XI.  BY-PRODUCTS  FROM  ASCLEPIAS  AND  APOCYNUM. 

If  financial  success  ever  attends  the  growing  of  any  native  extra- 
tropical  North  American  plant  on  a  commercial  scale  for  its  rubber, 
this  will  doubtless  be  due  in  part  to  other  products  derived  from  the 
same  crop.  The  most  promising  of  these  by-products,  as  far  as  the 
plants  especially  treated  in  this  report  are  concerned,  is  fiber.  It  is  a 
fortunate  circumstance  that  the  botanical  groups  which  include  the 
most  important  species  from  the  point  of  view  of  rubber-content  are 
also  characterized  by  the  presence  of  bast  fibers  suitable  to  the  manu- 
facture of  at  least  the  cheaper  grades  of  cloth  and  cordage.  If  not  so 
used,  this  fiber,  which  would  be  left  after  the  extraction  of  the  rubber, 
could  be  utilized  as  pulp  for  the  manufacture  of  paper. 

Fiber — The  cultivation  of  milkweeds,  especially  Asclepias  syriaca 
and  A.  incarnata,  for  their  fiber  has  been  advocated  from  time  to 
tune  by  enthusiasts,  some  of  whom  have  claimed  that  commercial 
success  was  assured.  But  the  actual  demonstration  has  never  been 
carried  through  in  this  country,  although  the  former  species  is  reported 
as  having  been  grown  for  its  fiber  in  Syria  and  as  far  north  as  Upper 
Silesia  (Dodge,  1897),  it  having  been  introduced  from  America.  It 
is  not  impossible  that,  although  the  cultivation  of  th«  plants  for  their 
fiber  alone  might  be  unprofitable,  the  utilization  of  the  fiber  after  these 
plants  had  been  grown  and  extracted  for  their  rubber  might  add  very 
largely  to  the  net  income  and  thus  would  be  established  an  industry 
based  upon  two  important  products.  In  this  case  two  prime  assump- 
tions will  be  necessary,  namely,  that  the  proper  time  of  harvesting 
for  the  two  products  will  coincide,  and  that  one  of  these  can  be  ex- 
tracted without  so  treating  the  material  as  to  make  the  recovery  of 
the  other  impossible. 

Certain  considerations  seem  to  render  these  assumptions  reasonably 
safe.  For  example,  it  is  generally  true  of  plants  yielding  bast  fibers, 
such  as  hemp  and  flax,  that  the  best  quality  and  yield  are  obtained  if 
they  are  harvested  at  about  the  time  of  maturity.  Such  plants  are 
often  gathered  when  the  leaves  begin  to  turn  yellow  or  when  the  seeds 
begin  to  ripen.  If  this  holds  also  for  milkweeds  and  Indian  hemp  it  is 
a  fortunate  circumstance,  for  while  there  is  not  a  universal  concordance 
in  results,  an  examination  of  the  tables  of  analyses  presented  in  this 
paper  will  show  that  milkweeds  gathered  when  fully  mature  usually 
carry  more  rubber  than  those  of  the  same  locality  gathered  earlier. 
Regarding  the  extraction  of  both  fiber  and  rubber  from  the  same  plant, 
all  that  can  be  said  is  that  experienced  rubber  manufacturers  who  have 
been  consulted  state  that  they  see  no  insuperable  difficulty  in  the  pro- 
posal. In  order  to  accomplish  this,  the  mill  should  be  so  constructed 
that  the  wet  plants  could  be  passed  between  rollers  that  would  crush  the 
stems  and  separate  the  fibers  without  breaking  them.  If  a  chemical 


BY-PRODUCTS   FROM   ASCELPIAS  AND   APOCYNUM.  57 

extraction  method  were  used  for  the  rubber  it  would  be  necessary  to 
consider  the  effect  of  this  upon  the  fiber.  The  solvents  commonly 
employed  would  not  injure  it. 

The  quality  of  the  fiber  in  the  plants  under  consideration  has  not 
been  thoroughly  tested.  It  is  quite  likely  that  this  will  vary  with  the 
different  botanical  species.  Perhaps  the  most  concise  published  state- 
ment on  the  subject  is  one  by  Dodge  (1897).  Speaking  of  the  common 
milkweed  (Asdepias  syriaca)  Dodge  says: 

"The  only  portion  of  the  plant  of  which  practical  use  can  be  made  is  the 
bast,  which  furnishes  quite  a  fine,  long,  glossy  fiber  that  is  strong  and  durable. 
Early  authorities  have  given  it  a  place  between  flax  and  hemp,  and  the  yield 
has  been  claimed  about  equal  to  the  latter.  Dr.  Schaeffer,  as  far  back  as  the 
fifties,  made  comparisons  of  the  two  fibers  in  Kentucky,  and  his  conclusions 
were  most  favorable  to  the  Asdepias  fiber.  The  native  fiber  was  taken  in 
winter  from  the  decayed  stalks  as  they  stood  in  the  ground  where  they  grew 
without  culture,  while  the  hemp  had  not  only  been  cultivated  but  treated 
afterwards  with  the  usual  care.  The  fiber  of  the  milkweed  was  nearly,  if 
not  quite,  as  strong  as  that  of  the  hemp,  but  apparently  finer  and  more 
glossy,  while  the  quantity  from  a  single  stalk  of  each  was  nearly  the  same." 

Samples  of  milkweed  fiber  have  been  examined  more  recently  by 
experts,  who  think  that  Dodge's  statement  is  rather  too  optimistic. 
While  these  later  opinions  were  not  based  upon  actual  tests  for  strength 
or  hi  weaving,  they  seem  fairly  reliable  and  lead  to  the  conclusion 
that  milkweed  fiber  is  not  equal  in  quality  to  hemp.  It  seems  to 
be  a  stiffer  fiber  that  does  not  spin  as  smoothly  or  as  evenly  as  the 
fibers  now  used,  so  that  new  methods  and  devices  would  need  to  be 
developed  for  handling  it.  The  milkweeds  referred  to  are  A.  incarnata 
and  A.  syriaca.  It  is  probable  that  there  is  some  variation  in  the 
properties  of  the  fiber  from  different  species. 

The  fiber  of  the  Indian  hemp  (Apocynum  cannabinum)  is  somewhat 
more  promising  than  that  of  the  milkweeds.  Dodge  (1897)  says  of  it : 

"Easily  separated  from  the  stalk,  and  when  cleaned  is  quite  fine,  long,  and 
tenacious.  In  color  it  is  light  cinnamon  as  usually  seen,  though  finely  pre- 
pared specimens  are  creamy  white  and  remarkably  fine  and  soft;  will  rank 
with  Asdepias  for  strength,  and  is  readily  obtained,  as  the  stems  are  long, 
straight,  smooth,  and  slender.  Although  paper  has  not  been  made  of  it,  it 
could  doubtless  be  utilized  for  the  purpose.  It  is  principally  employed  by 
the  North  American  Indians,  who  manufacture  from  it  in  rude  fashion,  bags, 
mats,  small  ornamental  baskets,  belts,  twine,  and  other  cordage,  fishing 
lines,  and  nets.  Among  fine  specimens  received  is  a  fish  line,  such  as  is  used 
by  the  Pai  Utes  at  the  Walker  River  Reservation  in  Nevada." 

Later  opinions  expressed  by  experts  who  have  examined  the  fiber 
are  that  Apocynum  fiber  might  have  a  commercial  value  if  it  could 
be  produced  at  a  cost  that  would  permit  it  to  compete  with  hemp  or 
jute.  The  fiber  of  this  plant  has  been  confused  by  some  authors  with 
that  of  the  Colorado  River  hemp  (Sesbania  macrocarpa),  an  entirely 
different  plant  of  the  Leguminosse. 


58  RUBBER-CONTENT   OF  NORTH  AMERICAN   PLANTS. 

Aside  from  the  bast  fibers  just  discussed,  both  Asclepias  and  Apocy- 
num  yield  a  surface  fiber  that  may  be  obtained  from  the  seed.  But  it  is 
unlikely  that  this  could  be  gathered  from  plants  grown  primarily  for 
their  rubber,  since  harvesting  would  probably  take  place  before  the 
fiber  would  have  time  to  develop,  and  aside  from  this  it  is  of  but  low 
value. 

Paper  pulp. — In  case  the  fiber  is  not  of  more  value  for  the  manu- 
facture of  cloth,  it  may  be  utilized  as  a  source  of  paper.  A  pulp  pre- 
pared from  one  of  the  eastern  milkweeds,  probably  Asclepias  syriaca, 
has  been  made  up  into  a  fair  quality  of  paper  at  one  of  the  leading 
factories.  This  was  done  many  years  ago  and  the  matter  dropped, 
since  it  was  not  profitable  to  grow  the  plants  for  this  purpose  alone. 
The  only  paper  made  from  milkweed  during  the  present  investigation 
was  manufactured  from  the  desert  species  (Asclepias  subulata)  by  the 
same  firm  and  the  product  found  to  be  much  superior  to  that  formerly 
examined.  The  plant  from  which  this  sample  came  was  collected  at 
Sentinel,  Arizona,  September  29, 1920,  under  No.  1054;  it  is  illustrated 
in  plate  1.  The  sample  consisted  of  the  very  hard,  dry  stems,  which 
were  practically  devoid  of  foliage,  as  is  generally  true  of  mature  stems. 
The  material  was  worked  up  by  the  factory,  following  the  usual 
procedure.  The  fiber  expert  for  the  company  states  that  the  report 
of  the  laboratory  is  very  encouraging,  that  the  milkweed  has  all  the 
appearances  of  making  a  good  pulp,  and  that  the  hand-made  sheets 
show  a  long,  soft  fiber  which  under  the  microscope  resembles  linen. 
The  processes  used  in  the  preparation  of  the  paper  were  as  follows: 

EXPERIMENT  1. 

1.  456  grams  of  material  as  received  cooked  with  10  per  cent  (45.6  grams) 

caustic  soda  for  3  hours  in  an  open  kettle. 

2.  This  material  run  through  wringer  rolls  to  crush  out. 

3.  Crushed  material  washed  clean  and  dried  out.     Weight  196  grams. 

4.  Material  thus  washed  next  cooked  in  closed  kettle  with  10  per  cent 

(19.6  grams)  lime  for  5  hours  at  30  pounds  pressure. 

5.  This  cooked  material  washed  and  beaten  in  beater  and  run  out  on 

hand  moulds.     151  grams  of  unbleached  paper  obtained. 

6.  This  151  grams  unbleached  paper  next  bleached  with  bleach  solution 

made  from  an  amount  of  bleach  powder  equal  to  7  per  cent 

by  weight  of  the  151  gra  ms  of  the  unbleached  fiber.    This  gave 

130  grams  of  bleached  paper. 
Yields: 

From  material  as  received  to  washed,  crushed,  bone-dry  fiber  (end 

of  operation  3),  43  per  cent. 
From  material  as  received  to  unbleached  paper  (end  of  operation  5), 

33  per  cent. 
From  material  as  received  to  bleached  paper  (end  of  operation  6), 

28.5  per  cent. 


BY-PRODUCTS   FROM   ASCLEPIAS   AND   APOCYNUM.  59 

EXPERIMENT  2. 

A  quantity  of  milkweed  stalks,  selected  as  described  in  Experiment  1, 
was  boiled  in  an  open  kettle  with  water  for  8  hours.  Softened  stalks  were 
crushed  in  wringer  as  above  described.  Then  furnished  to  autoclave,  where 
they  were  cooked  with  10  per  cent  caustic  soda  for  8  hours  at  30  pounds 
pressure.  The  cooked  material  was  washed,  beaten,  and  run  out  into  hand 
sheets.  The  yield  from  material  as  received  to  bone-dry  paper,  unbleached, 
is  40  per  cent. 

The  paper  resulting  from  this  experiment  was  highly  satisfactory;  quite  as 
satisfactory  as  that  obtained  from  Experiment  1,  and  obtained  in  a  much 
less  expensive  manner. 

It  was  thought  by  cooking  with  lime  a  paper  of  equally  soft,  pliable  quality 
might  be  obtained.  The  result  of  this  attempt  we  report  under  Experiment  3. 

EXPERIMENT  3. 

A  quantity  of  selected  milkweed  stalks  was  boiled  in  water  for  8  hours, 
crushed  in  the  wringer,  and  cooked  in  the  autoclave  with  10  per  cent  lime 
for  8  hours,  at  30  pounds  pressure.  Cooked  stock  was  washed  clean  and 
beaten,  then  run  out  in  hand  molds  as  sheets  of  paper. 

Yield  from  material  as  received  to  bone-dry  paper  42  per  cent. 

The  resulting  paper  from  Experiment  3  was  much  inferior  in  softness  and 
pliability  to  that  obtained  in  either  Experiment  1  or  2,  although  the  yield 
from  material  as  received  to  unbleached  paper  was  the  highest  reported. 
It  is,  however,  relatively  free  from  harsh,  incrusted,  shivey  particles.  Paper 
obtained  from  Experiments  \  and  2  was  entirely  free  from  harsh,  shivey 
particles. 

The  lime  used  in  these  experiments  was  a  good  grade  of  hydrated 
lime,  containing  about  95  per  cent  calcium  hydrate,  1.3  per  cent 
impurities  (silicon  dioxide,  iron,  and  alumina).  The  caustic  soda  was 
good  grade  58  Solvay  process  caustic. 

The  resulting  fibers,  as  obtained  from  sheets  of  paper  made  in  Exper- 
ments  1  and  2,  have  many  of  the  characteristics  of  linen.  They  are 
long  and  silky,  possessing  remarkable  strength  for  the  type  of  material 
from  which  they  are  obtained. 

It  is  of  interest  to  note  that  this  material  does  not  yield  readily  to 
bleaching  with  calcium  hypochlorite.  It  may,  however,  yield  more 
readily  to  bleaching  with  hypochlorous  acid  or  possibly  sodium- hypo- 
chlorite. 

For  material  of  this  type,  yields  for  material  as  received  to  un- 
bleached paper  in  all  cases  are  exceptionally  high. 

An  interesting  note  is  made  by  the  laboratory  expert  that  very 
likely  in  the  process  of  extracting  rubber  the  raw  material  may  be 
put  through  a  breaking-up  process  which  would  so  separate  the  woody, 
fibrous,  and  pithy  components  as  to  render  unnecessary  the  pre- 
liminary cook  with  caustic  soda.  The  paper-maker  therefore  would 
start  with  the  lime  cook,  thus  saving  the  prohibitive  cost  of  reduction 
by  caustic  and  crushing. 

This  matter  has  been  taken  up  also  with  a  director  of  a  scientific 
laboratory  connected  with  one  of  the  large  rubber  factories,  and  he  is 


60  RUBBER-CONTENT   OP   NORTH   AMERICAN  PLANTS. 

of  the  opinion  that  there  will  be  no  insurmountable  difficulty  in  so 
constructing  the  rubber  machinery  as  thoroughly  to  separate  the  fiber 
without  injuring  it.  It  seems  reasonably  certain,  therefore,  that  not 
only  could  both  rubber  and  fiber  be  extracted  from  the  same  material 
in  a  practical  way,  but  that  the  extraction  of  the  rubber  would  permit 
the  paper  manufacturer  to  proceed  with  considerably  less  expense 
than  if  he  were  to  use  the  plant  as  it  comes  from  the  field. 

In  view  of  the  above  favorable  reports,  it  is  now  proposed  to  collect 
a  half -ton  sample  of  the  desert  milkweed  in  order  that  the  rubber  may 
be  extracted  and  the  residue  turned  over  to  the  paper  manufacturers 
for  further  experimentation  and  report. 

XII.  AGRICULTURAL  POSSIBILITIES  OF  ASCLEPIAS  AND 

APOCYNUM. 

If  natural  rubber  is  ever  produced  in  commercial  quantities  in  the 
United  States,  it  will  be  from  a  plant  which  will  give  large  yields  on 
cheap  land,  and  one  which  can  be  handled  almost  entirely  by  ma- 
chinery. This  is  assuming  that  labor  and  other  economic  conditions  will 
remain  about  as  they  now  are.  The  reason  for  these  requirements  is 
the  realization  that  North  American  rubbers,  unless  possessing  unique 
qualities  that  would  especially  fit  them  for  special  uses,  would  need  to 
compete  in  the  market  with  the  foreign  product.  The  price  of  the  best 
imported  grades  is  now  down  to  about  25  cents  per  pound,  and  even 
this  will  doubtless  be  lowered  with  the  improvement  of  plantation 
methods  and  when  the  present  voluntary  restrictions  on  output  are 
removed.  Another  quality  which  it  may  be  necessary  for  the  Ameri- 
can-grown plant  to  possess  is  the  ability  to  yield  by-products.  It 
seems  quite  possible,  for  example,  that  while  the  species  of  plants  con- 
sidered in  this  report  might  never  be  made  to  produce  a  suitable  return 
from  their  rubber  alone,  yet  their  cultivation  might  become  a  profitable 
industry  in  view  of  the  additional  returns  to  be  expected  from  the 
sale  of  the  pulp  after  the  rubber  is  extracted. 

Of  all  the  plants  here  considered,  the  desert  milkweed  (Asclepias 
subulata)  apparently  comes  nearest  to  fulfilling  the  requirements 
above  set  forth.  It  is  a  perennial  that  grows  rapidly  on  poor  desert 
lands  with  a  minimum  of  moisture,  is  of  such  a  nature  as  to  permit  of 
harvesting  by  machinery,  reproduces  after  cutting,  and  yields  a  paper- 
pulp  of  better  quality  than  that  from  other  milkweeds.  The  per- 
centage of  rubber  present  averages  about  as  high  as  that  of  the  others 
and  is  sufficiently  fluctuating  to  indicate  that  superior  high-yielding 
strains  could  be  produced  through  breeding  and  selection.  This 
building-up  of  the  rubber-content  will  be  an  essential  preliminary  to 
the  utilization  of  any  of  the  plants  here  considered.  It  would  be 
futile  to  attempt  the  profitable  production  of  rubber  on  the  basis  of 
the  percentage  content  thus  far  found  in  American  species. 


AGRICULTURAL  POSSIBILITIES  OF  ASCLEPIAS  AND  APOCYNUM.    61 

Before  a  fair  idea  can  be  formed  as  to  the  yields  to  be  obtained,  it 
will  be  necessary  to  carry  out  some  experiments  in  growing  the  plants 
under  field  conditions.  In  the  meantime,  a  study  of  the  wild  plants 
and  of  the  tables  of  analyses  may  give  some  hint  as  to  what  may  be 
expected.  It  is  known,  for  example,  that  under  favorable  conditions 
individual  plants  of  the  desert  milkweed  (Asclepias  suhulata)  weigh 
15  to  80  pounds.  Perhaps  30  pounds,  green  weight,  is  a  fair  average 
to  expect  when  tillage  and  other  methods  to  conserve  moisture  are 
practiced.  Such  plants  have  a  spread  of  3  to  6  feet  and  require  perhaps 
3  years  to  mature,  after  which  annual  crops  presumably  could  be  har- 
vested. It  is  even  possible  that  2  or  more  crops  could  be  gathered  in 
one  year,  as  is  practiced  with  alfalfa,  but  the  effect  of  frequent  cropping 
on  the  vigor  of  the  plant  is  not  known.  Irrigation,  even  in  slight 
amount,  would  certainly  increase  the  tonnage  as  well  as  permit  closer 
planting  and  more  frequent  cropping.  It  might  also  modify,  either 
one  way  or  the  other,  the  percentage  of  rubber  produced. 

In  carrying  out  estimates  an  allowance  should  be  made  for  about 
60  per  cent  of  water  in  the  green  stems.  But  the  determining  factor 
is  the  percentage  of  rubber  present,  and  no  definite  prediction  can  be 
made  as  to  what  this  will  be.  The  average  content  of  9  unimproved 
plants  growing  at  Sentinel,  Arizona,  exclusive  of  the  woody  base, 
was  3.1  per  cent.  With  this  and  the  above  figures  as  to  size  and  other 
factors  as  a  basis,  it  would  seem  that  between  300  and  500  pounds  of 
rubber  per  acre  might  be  expected  from  each  crop  in  case  a  uniform 
stand  of  fair-sized  plants-  can  be  secured.  If  it  is  assumed  that  the 
highest  percentage  found  in  an  individual  plant  could  be  uniformly 
maintained  under  methods  of  seed  selection,  then  these  figures  could 
be  at  least  doubled.  Notable  success  in  the  improvement  of  other 
agricultural  crops  through  breeding  combined  with  selection  justifies 
the  hope  that  the  average  of  3.1  per  cent  can  be  increased  to  several 
times  this  amount,  with  a  corresponding  increase  in  the  total  yield  per 
acre.  Such  improvement  would  be  necessary  as  a  preliminary  to  the 
profitable  cultivation  of  the  desert  milkweed  for  its  rubber  and  fiber. 

In  actual  practice  the  yield  would  be  considerably  modified  through 
the  inclusion  of  an  undeterminable  proportion  of  resins  and  other  sub- 
stances. The  amount  of  such  matter  varies  from  about  6  per  cent  in 
the  best  Para  to  about  35  per  cent  in  guayule,  and  even  higher  in 
some  inferior  Africas. 

To  the  income  of  the  rubber  is  to  be  added  the  value  of  the  pulp 
for  paper  manufacture.  As  already  demonstrated  (p.  58)  the  air- 
dry  stems  yield  about  43  per  cent  of  bone-dry  fiber,  or  28.5  per  cent  of 
bleached  paper.  No  figures  have  been  obtained  as  to  the  value  of  the 
pulp,  but  it  would  be  higher  than  that  made  from  hemp  and  similar 
plants,  since  the  separation  of  the  fibers  incident  to  rubber  extraction 
would  be  such  as  to  render  unnecessary  certain  preliminary  processes. 


62  KUBBER-CONTENT   OF  NORTH   AMERICAN  PLANTS. 

It  may  be  found  on  further  examination  that  other  species  than 
Asclepias  subulata  are  suited  to  the  purposes  under  consideration. 
A.  sullivanti,  for  example,  carries  a  higher  percentage  of  rubber  in 
the  leaves  alone  than  is  found  in  the  stems  of  the  leafless  A.  subulata, 
and  the  plants  are  very  tall,  thus  yielding  a  large  tonnage.  But 
usually  its  stems  are  very  poor  yielders  and  as  far  as  known  the  plants 
grow  only  in  fairly  good  soil.  The  fiber  would  be  of  value,  but  is 
probably  of  lower  grade  than  that  of  the  desert  species.  If]  A .  sullivanti 
can  be  induced  to  grow  on  cheap  land,  or  if  the  rubber-content  can 
be  very  materially  increased,  particularly  in  the  stems,  it  would  have 
a  chance  as  a  possible  rubber  plant,  occupying  its  own  geographic 
area.  Other  species  to  be  considered  are  the  ones  given  special  con- 
sideration in  this  report.  Their  qualifications,  as  far  as  known,  are 
briefly  discussed  in  each  case  under  the  subheadings  of  "Distribution 
and  ecology"  and  "Rubber-content."  It  also  should  be  remembered 
that  only  a  few  of  the  native  latex  plants  have  been  studied.  Possibly 
there  are  others  which  on  examination  would  be  found  to  hold  out 
much  more  promise  than  any  of  these. 

It  is  readily  seen  from  the  above  statements  that  a  much  more 
extended  and  detailed  investigation  will  be  necessary  before  anything 
definite  can  be  said  regarding  the  possibilities  of  these  plants  as 
agricultural  crops.  The  present  study  was  undertaken  chiefly  to 
determine  whether  or  not  such  investigations  could  be  carried  forward 
with  some  prospect  of  ultimate  success.  It  seems  to  the  authors  that 
the  facts  thus  far  established  amply  demonstrate  the  desirability  and 
wisdom  of  instituting  further  studies  and  experiments.  In  case  these 
are  carried  out,  they  should  be  extended  to  include  Chrysothamnus 
and  other  plants  not  treated  in  detail  in  this  paper  and  should  aim  to 
cover  the  following  points: 

1.  Further  field  studies  of  wild  plants  to  discover  better  species  and 

strains  than  those  already  found. 

2.  Experiments  in  breeding  and  selection. 

3.  Modifications  in  environment,  especially  the  water-  and  alkali  relations, 

and  their  effect  upon  growth  and  yield. 

4.  Manipulation  of  the  plant  itself,  with  the  object  of  increasing  the 

amount  of  rubber-bearing  tissue.  This  is  already  under  way  for 
chrysil. 

5.  Field  experiments  to  determine  the  tonnage  per  acre  to  be  expected, 

and  the  best  methods  of  culture. 

6.  Studies  in  the  seasonal  formation  of  rubber  in  the  plant.     This  includes 

a  combination  of  the  most  refined  chemical  methods,  with  micro- 
scopic studies  of  cell-contents  and  their  changes  precedent  to  and 
during  the  deposition  of  rubber. 

7.  Quality  and  value  of  the  product.     Investigations  along  this  line  are 

already  in  progress.  ChrysU  and  several  species  of  Asclepias 
rubber  will  be  studied. 

8.  By-products,  including  fiber,  pulp  for  paper,  acetone,  alcohol,  dyes, 

potash,  essential  oils,  waxes,  and  resins. 


SUMMARY    AND    CONCLUSIONS.  63 

XIII.    SUMMARY  AND  CONCLUSIONS. 

(1)  The  purpose  of  the  investigation  was  to  make  a  preliminary 
study  of  native  North  American  plants  in  regard  to  their  rubber- 
content.    While  this  is  a  matter  of  general  scientific  interest,  it  was 
hoped  that  some  species  would  be  found  that  carried  enough  rubber 
to  justify  their  further  study  with  a  view  to  establishing  a  rubber- 
growing  industry  within  the  United  States.    This  is  desirable  in  that  it 
would  utilize  land  now  lying  idle,  and,  furthermore,  would  render  the 
nation  to  some  extent  independent  of  overseas  importations. 

(2)  The   species   examined  were  mostly  latex  plants,   especially 
species  of  Asclepias  (milkweeds)  and  Apocynum  (Indian  hemp).    The 
work  is  a  continuation  of  earlier  studies  on  native  shrubs.    A  total 
of  about  225  North  American  species  have  now  been  examined  and  a 
record  made  as  to  the  presence  or  absence  of  rubber  in  their  tissues. 
About  6  species  of  shrubs  and  16  species  of  latex-bearing  herbs  are 
considered  worthy  of  further  study  and  experiment.     Nothing  is 
known  concerning  the  rubber-content  of  about  80  other  native  species 
of  Asclepias,  all  of  which  presumably  contain  at  least  a  small  amount 
of  rubber. 

(3)  The  researches  of  earlier  workers  on  Asclepias  and  Apocynum 
revealed  but  very  small  amounts  of  rubber.    By  carrying  the  examina- 
tions to  other  species,  and  to  other  ecologic  and  genetic  forms  of  the 
same  species,  much  larger  percentages  in  the  plant  have  been  found. 

(4)  The  acetone-benzene  method  of  analysis  was  employed.  The  per- 
centages reported  are  therefore  for  rubber,  practically  free  from  resin  or 
other  admixtures.     They  are  based  upon  the  dry  weight  of  the  material 
analyzed. 

(5)  The  species  of  Asclepias  and  Apocynum  reported  as  of  special 
interest  are  all  perennials.     Some  make  a  good  growth  on  land  so 
poor  or  dry  that  it  is  not  at  present  utilized.     They  grow  rapidly 
from  seed  and  from  portions  of  the  root.     It  is  possible  that  several 
crops  could  be  obtained  in  one  year  without  replanting.     The  crop 
could  be  handled  almost  entirely  by  machinery. 

(6)  The  amount  of  rubber  in  the  plant  appears  to  vary  with  the 
species,  with  small  races,  or  strains  within  each  species,  and  also  with 
the  ecologic  conditions  obtaining  during  the  period  of  growth.     High 
percentages  are  often  associated  with  vigor  of  growth.     Mature  herb- 
age often  carries  higher  percentages  than  young  herbage. 

(7)  The  latex  plants  examined  usually  carry  much  higher  per- 
centages in  the  leaves  than  in  the  stems  or  roots.    The  one  exception 
is  a  nearly  leafless  desert  milkweed  (Asclepias  subulata),  the  stems  of 
which  contain  from  2  to  6.4  per  cent.     This  is  a  species  of  outstanding 
promise.     Others  considered  as  of  special  interest,  with  the  rubber- 
content  of  their  mature  leaves,  are: 


64  RUBBER-CONTENT   OF   NORTH   AMERICAN   PLANTS. 

Per  cent.  Per  cent. 

Asclepias  sullivanti 1.2  to  8.1        Asclepias  galioides 0.6  to  5.2 

syriaca 1.1  to  4.4  brachystephana 2.1  to  2.9 

californica 2.6  to  4.3  speciosa 1     to  3 

latifolia 2     to  3.8        Apocynum  cannabinum 0.7  to  5.1 

mexicana 1.4  to  4J8 

(8)  The  product  has  been  examined  for  only  a  few  species,  and  these 
examinations  have  not  been  thorough.     They  indicate,  however,  that 
the  rubber  is  probably  of  low  grade,  not  to  be  compared  with  fine 
imported  Para,  with  plantation  rubber,  nor  with  chrysil.     It  would 
doubtless  find  use  for  mixing  with  other  rubbers  and  in  the  manufac- 
tures where  great  elasticity  is  not  demanded. 

(9)  A  marked  variability  in  the  wild  plants,  and  the  number  of 
species  involved,  indicate  that  strains  with  a  higher  rubber-content 
than  any  thus  far  discovered  could  be  developed  by  breeding  and  then 
perpetuated  either  by  using  pedigreed  seed  or  by  vegetative  repro- 
duction.    It  is  possible  that  improvement  in  the  quality  of  the  rubber 
can  also  be  effected  by  such  methods. 

(10)  By-products  in  the  form  of  fiber  and  paper-pulp  can  be  obtained 
from  the  residue  after  the  extraction  of  the  rubber.     The  pulp  of 
Asclepias  subulata  has  been  worked  up  into  paper  of  good  quality, 
the  yield  of  dry  fiber  being  43  per  cent  and  of  bleached  paper  28.5 
per  cent  of  the  air-dry  weight  of  the  stems.     The  previous  extraction 
of  rubber  reduces  the  cost  of  paper-making.     It  seems  not  unlikely 
that  for  this  species  the  profits  from  fiber  would  exceed  those  from 
rubber. 

(11)  No  opinion  is  expressed  regarding  the  financial  results  that 
might  follow  from  an  attempt  to  grow  the  plants  on  a  commercial 
scale.     It  is  certain,  however,  that  considerable  scientific  experiment 
should  precede  any  such  attempt.     Sufficient  data  have  been  accumu- 
lated to  justify  the  recommendation  that  these  experiments  be  now 
undertaken.     In  this  connection  there  should  be  considered  those 
native  shrubs  in  which  the  rubber  occurs  as  solid  particles  and  also 
various  exotic  shrubs  and  herbs,  as  well  as  the  native  species  of  Asclepias 
and  Apocynum. 


LITERATURE    CITED.  65 

XIV.  LITERATURE  CITED. 

COCKERELL,  T.  D.  A.     1903.    The  Colorado  rubber  plant.     Bull.  Colo.  Coll.  Mus.  1: 1. 
COCKERELL,  W.  P.     1904.     Note  on  a  rubber-producing  plant.     Science,  II,  19:  314. 
COOK,  O.  F.     1903.     Culture  of  the  Central  American  rubber  tree.      U.  S.  Dept.  Agr. 

Bureau  of  Plant  Industry  Bull.  49. 
DODGE,  C.  R.     1897.     Descriptive  catalogue  of  useful  fiber  plants  of  the  world.    U.  S. 

Dept.  Agr.  Fiber  Investig.  Rept.  9. 

DUNSTAN,  W.  R.     1910.    Editorial  in  Imp.  Inst.  Bull.  8:  48. 
Fox,  C.  P.     1911.    Ohio-grown  rubber,  crop  of  1910.    Ohio  Nat.,  11:  271. 

.     1912.     Another  Ohio-grown  rubber.     Ibid.,  12:  469. 

HALL,  H.  M.,  and  T.  H.  GOODSPEED.     1919.     Rubber-plant  survey  of  Western  North 

America.     Univ.  Calif.  Pub.,  Botany,  7: 159-278. 
and  H.  G.  YATES.     1915.    Stock-poisoning  plants  of  California.     Calif.  Agr.  Exp. 

Sta.  Bull.  249. 
HARRIES,  C.  1905.    Zur  Kenntniss  der  Kautschukarten.    Ber.  Deut.  Chem.  Ges.  38: 

1195-1203. 

.     1919.    Untersuchungen  iiber  den  naturlichen  und  kunstlichen  Kautschukarten. 

HILLIER,  J.  M.     1906.     Colorado  rubber.     Bull.  Misc.  Inform.  Kew,  218-219. 
HOAGLAND,  D.  R.   1915.   Organic  constituents  of  Pacific  Coast  kelps.   Journ.  Agr.  Research 

4:47. 

LLOYD,  F.  E.     1911.     Guayule.    Carnegie  Inst.  Wash.  Pub.  139. 
MARSH,  C.  D.,  A.  B.  CLAWSON,   J.  F.  COUCH,  and  W.  W.  EGGLESTON.     1920.      The 

whorled  milkweed  as  a  poisonous  plant.    U.  S.  Dept.  Agr.  Bull.  800. 
PEARSON,  H.  C.     1919.    Rubber-producing  weeds  in  Germany.     India  Rubber  World, 

59:201. 

.     1920.     Ocotillo  again  to  the  front.     Ibid.,  63:  9. 

RUSBY,  H.  H.     1909.    Rubber  plants  of  Mexico.    Torreya,  9: 177-184. 
SAUNDERS,  A.  T.     1910.    Rubber  found  in  milkweed.     India  Rubber  World,  43:  4. 
SAUNDEHS,  WILLIAM.     1875.    On  the  manufacture  of  rubber  from  a  milkweed.     Am. 

Pharm.  Assoc.  Proc.,  23:655-658. 


HALL  AND  LONG 


Asclepias  subulata,  collected  at  Sentinel,  Arizona.     Plant  5  feet  high. 


HALL  AND  LONG 


PLATE  2 


A.  Community  of  Asclepias  subulata  in  an  arroyo  near  Mesa,  Arizona. 

B.  Individual  plant  of  A.  subulata,  showing  maximum  size  in  nature. 


HALL  AND   LONG 


A.  Asclepias  sullivanti,  collected  at  Lincoln,  Nebraska. 

B.  Asclepias  mexicana,  growing  near  Dos  Palos,  California. 

C.  Community  of  A.  mexicana,  near  Dos  Palos.    The  plant  in  the  foreground  is  6  feet  high. 


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